Reland of Break page load metrics test dependency on IPC.

Reland of Break page load metrics test dependency on IPC. (patchset #1 id:1 of https://codereview.chromium.org/2847803002/ )

Reason for revert:
The flake that caused the previous revert by zmin existed before the original patch landed, so the original patch is not the cause of the flake.

This refactor should help us to un-flake the test, so I'm re-landing it.

Original issue's description:
> Revert of Break page load metrics test dependency on IPC. (patchset #2 id:20001 of https://codereview.chromium.org/2847513002/ )
> 
> Reason for revert:
> PageLoadMetricsBrowserTest.DocumentWriteReloada is flaky
> 
> https://luci-milo.appspot.com/buildbot/chromium.memory/Linux%20MSan%20Tests/387
> 
> Original issue's description:
> > Break page load metrics test dependency on IPC.
> > 
> > This change breaks direct dependencies on IPC for page load metrics
> > tests, in order to ease the transition to mojo.
> > 
> > Previously, unit tests exercised the IPC dispatch path. This
> > achieved slightly increased coverage but coupled unit tests to
> > IPC. We now dispatch simulated timing events directly to callbacks,
> > bypassing the IPC dispatch.
> > 
> > Additionally, we break the browsertest dependency on IPC and add
> > a MetricsWebContentsObserver::TestingObserver, which can observe
> > state changes at the observer level, instead of watching for IPC
> > messages. This both simplifies the logic and more directly verifies
> > expected behavior at the appropriate level.
> > 
> > BUG=715744
> > TBR=csharrison
> > 
> > Review-Url: https://codereview.chromium.org/2847513002
> > Cr-Commit-Position: refs/heads/master@{#467687}
> > Committed: https://chromium.googlesource.com/chromium/src/+/9567f9fa3fcfcaa50fc7b6a6adf66106a2eb6be5
> 
> TBR=lpy@chromium.org,bmcquade@chromium.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=715744
> 
> Review-Url: https://codereview.chromium.org/2847803002
> Cr-Commit-Position: refs/heads/master@{#467691}
> Committed: https://chromium.googlesource.com/chromium/src/+/2cb37fe0beb13e6baa8852662362e9204d1ebfa3

TBR=lpy@chromium.org,zmin@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=715744
Review-Url: https://codereview.chromium.org/2849563002
Cr-Commit-Position: refs/heads/master@{#467695}
Committed: https://chromium.googlesource.com/chromium/src/+/44d3a319f57f02f92beb9ceb0ad973d7d4d39c82

[Bryan McQuade, 2017-04-27 15:52:14]

The CQ bit was checked by bmcquade@chromium.org

[Bryan McQuade, 2017-04-27 15:52:14]

Created Reland of Break page load metrics test dependency on IPC.

[commit-bot: I haz the power, 2017-04-27 15:52:44]

CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-04-27 15:53:11]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1493308334111700, "parent_rev":
"bb2388b6cee45ca8cba764245b2886727b3fafb8", "commit_rev":
"44d3a319f57f02f92beb9ceb0ad973d7d4d39c82"}

[commit-bot: I haz the power, 2017-04-27 15:53:27]

Description was changed from

==========
Reland of Break page load metrics test dependency on IPC. (patchset #1 id:1 of
https://codereview.chromium.org/2847803002/ )

Reason for revert:
The flake that caused the previous revert by zmin existed before the original
patch landed, so the original patch is not the cause of the flake.

This refactor should help us to un-flake the test, so I'm re-landing it.

Original issue's description:
> Revert of Break page load metrics test dependency on IPC. (patchset #2
id:20001 of https://codereview.chromium.org/2847513002/ )
> 
> Reason for revert:
> PageLoadMetricsBrowserTest.DocumentWriteReloada is flaky
> 
>
https://luci-milo.appspot.com/buildbot/chromium.memory/Linux%20MSan%20Tests/387
> 
> Original issue's description:
> > Break page load metrics test dependency on IPC.
> > 
> > This change breaks direct dependencies on IPC for page load metrics
> > tests, in order to ease the transition to mojo.
> > 
> > Previously, unit tests exercised the IPC dispatch path. This
> > achieved slightly increased coverage but coupled unit tests to
> > IPC. We now dispatch simulated timing events directly to callbacks,
> > bypassing the IPC dispatch.
> > 
> > Additionally, we break the browsertest dependency on IPC and add
> > a MetricsWebContentsObserver::TestingObserver, which can observe
> > state changes at the observer level, instead of watching for IPC
> > messages. This both simplifies the logic and more directly verifies
> > expected behavior at the appropriate level.
> > 
> > BUG=715744
> > TBR=csharrison
> > 
> > Review-Url: https://codereview.chromium.org/2847513002
> > Cr-Commit-Position: refs/heads/master@{#467687}
> > Committed:
https://chromium.googlesource.com/chromium/src/+/9567f9fa3fcfcaa50fc7b6a6adf6...
> 
> TBR=lpy@chromium.org,bmcquade@chromium.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=715744
> 
> Review-Url: https://codereview.chromium.org/2847803002
> Cr-Commit-Position: refs/heads/master@{#467691}
> Committed:
https://chromium.googlesource.com/chromium/src/+/2cb37fe0beb13e6baa8852662362...

TBR=lpy@chromium.org,zmin@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=715744
==========

to

==========
Reland of Break page load metrics test dependency on IPC. (patchset #1 id:1 of
https://codereview.chromium.org/2847803002/ )

Reason for revert:
The flake that caused the previous revert by zmin existed before the original
patch landed, so the original patch is not the cause of the flake.

This refactor should help us to un-flake the test, so I'm re-landing it.

Original issue's description:
> Revert of Break page load metrics test dependency on IPC. (patchset #2
id:20001 of https://codereview.chromium.org/2847513002/ )
> 
> Reason for revert:
> PageLoadMetricsBrowserTest.DocumentWriteReloada is flaky
> 
>
https://luci-milo.appspot.com/buildbot/chromium.memory/Linux%20MSan%20Tests/387
> 
> Original issue's description:
> > Break page load metrics test dependency on IPC.
> > 
> > This change breaks direct dependencies on IPC for page load metrics
> > tests, in order to ease the transition to mojo.
> > 
> > Previously, unit tests exercised the IPC dispatch path. This
> > achieved slightly increased coverage but coupled unit tests to
> > IPC. We now dispatch simulated timing events directly to callbacks,
> > bypassing the IPC dispatch.
> > 
> > Additionally, we break the browsertest dependency on IPC and add
> > a MetricsWebContentsObserver::TestingObserver, which can observe
> > state changes at the observer level, instead of watching for IPC
> > messages. This both simplifies the logic and more directly verifies
> > expected behavior at the appropriate level.
> > 
> > BUG=715744
> > TBR=csharrison
> > 
> > Review-Url: https://codereview.chromium.org/2847513002
> > Cr-Commit-Position: refs/heads/master@{#467687}
> > Committed:
https://chromium.googlesource.com/chromium/src/+/9567f9fa3fcfcaa50fc7b6a6adf6...
> 
> TBR=lpy@chromium.org,bmcquade@chromium.org
> # Skipping CQ checks because original CL landed less than 1 days ago.
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=715744
> 
> Review-Url: https://codereview.chromium.org/2847803002
> Cr-Commit-Position: refs/heads/master@{#467691}
> Committed:
https://chromium.googlesource.com/chromium/src/+/2cb37fe0beb13e6baa8852662362...

TBR=lpy@chromium.org,zmin@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=715744

Review-Url: https://codereview.chromium.org/2849563002
Cr-Commit-Position: refs/heads/master@{#467695}
Committed:
https://chromium.googlesource.com/chromium/src/+/44d3a319f57f02f92beb9ceb0ad9...
==========

[commit-bot: I haz the power, 2017-04-27 15:53:28]

Committed patchset #1 (id:1) as
https://chromium.googlesource.com/chromium/src/+/44d3a319f57f02f92beb9ceb0ad9...