Revert of Remove the is_loading_ field from WebContentsImpl

Revert of Remove the is_loading_ field from WebContentsImpl (patchset #24 id:460001 of https://codereview.chromium.org/1545973002/ )

Reason for revert:
CHECK(is_loading_) appears to fail (flakily?) on  Linux ChromiumOS Tests (1). More details in the bug.

Original issue's description:
> Remove the is_loading_ field from WebContentsImpl
>
> This CL removes the is_loading_ field from WebContents, in favor of only
> tracking the loading state through the FrameTree. Currently the loading state
> is tracked in both, leading to more complexity in the code.
>
> BUG=571887, 298193
> CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_site_isolation
>
> Committed: https://crrev.com/db73eb6cf1e59e753fb53e8c744620b00606ccd5
> Cr-Commit-Position: refs/heads/master@{#374651}

TBR=rockot@chromium.org,carlosk@chromium.org,nasko@chromium.org,nick@chromium.org,clamy@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=571887, 298193
Committed: https://crrev.com/1c12bf40e35ffd32c6eadbf1503155b43e31bb97
Cr-Commit-Position: refs/heads/master@{#374680}

[waffles, 2016-02-10 17:41:02]

Created Revert of Remove the is_loading_ field from WebContentsImpl

[commit-bot: I haz the power, 2016-02-10 17:41:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1690633002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1690633002/1

[commit-bot: I haz the power, 2016-02-10 17:45:59]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-02-10 17:46:53]

Description was changed from

==========
Revert of Remove the is_loading_ field from WebContentsImpl (patchset #24
id:460001 of https://codereview.chromium.org/1545973002/ )

Reason for revert:
CHECK(is_loading_) appears to fail (flakily?) on  Linux ChromiumOS Tests (1).
More details in the bug.

Original issue's description:
> Remove the is_loading_ field from WebContentsImpl
> 
> This CL removes the is_loading_ field from WebContents, in favor of only
> tracking the loading state through the FrameTree. Currently the loading state
> is tracked in both, leading to more complexity in the code.
> 
> BUG=571887, 298193
> CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_site_isolation
> 
> Committed: https://crrev.com/db73eb6cf1e59e753fb53e8c744620b00606ccd5
> Cr-Commit-Position: refs/heads/master@{#374651}

TBR=rockot@chromium.org,carlosk@chromium.org,nasko@chromium.org,nick@chromium...
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=571887, 298193
==========

to

==========
Revert of Remove the is_loading_ field from WebContentsImpl (patchset #24
id:460001 of https://codereview.chromium.org/1545973002/ )

Reason for revert:
CHECK(is_loading_) appears to fail (flakily?) on  Linux ChromiumOS Tests (1).
More details in the bug.

Original issue's description:
> Remove the is_loading_ field from WebContentsImpl
>
> This CL removes the is_loading_ field from WebContents, in favor of only
> tracking the loading state through the FrameTree. Currently the loading state
> is tracked in both, leading to more complexity in the code.
>
> BUG=571887, 298193
> CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_site_isolation
>
> Committed: https://crrev.com/db73eb6cf1e59e753fb53e8c744620b00606ccd5
> Cr-Commit-Position: refs/heads/master@{#374651}

TBR=rockot@chromium.org,carlosk@chromium.org,nasko@chromium.org,nick@chromium...
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=571887, 298193

Committed: https://crrev.com/1c12bf40e35ffd32c6eadbf1503155b43e31bb97
Cr-Commit-Position: refs/heads/master@{#374680}
==========

[commit-bot: I haz the power, 2016-02-10 17:46:54]

Patchset 1 (id:??) landed as
https://crrev.com/1c12bf40e35ffd32c6eadbf1503155b43e31bb97
Cr-Commit-Position: refs/heads/master@{#374680}