M57: Add UseCounter for ServiceWorkerNavigationPreload.

M57: Add UseCounter for ServiceWorkerNavigationPreload.

We have a UMA already (ServiceWorker.NavigationPreload.HeaderSize) which gives
the total count, but it's hard to compute the % of page views without a
UseCounter.

BUG=649558
Review-Url: https://codereview.chromium.org/2659043002
Cr-Commit-Position: refs/heads/master@{#448529}
(cherry picked from commit d688b918acfbbf4572db430fefeea3374286baf1)

Review-Url: https://codereview.chromium.org/2684933011 .
Cr-Commit-Position: refs/branch-heads/2987@{#429}
Cr-Branched-From: ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}
Committed: https://chromium.googlesource.com/chromium/src/+/c3c0fd9fbf6ae5659395b941b80dc142e5ff83b9

[falken, 2017-02-10 05:32:41]

Description was changed from

==========
M57: Add UseCounter for ServiceWorkerNavigationPreload.

We have a UMA already (ServiceWorker.NavigationPreload.HeaderSize) which gives
the total count, but it's hard to compute the % of page views without a
UseCounter.

BUG=649558

Review-Url: https://codereview.chromium.org/2659043002
Cr-Commit-Position: refs/heads/master@{#448529}
(cherry picked from commit d688b918acfbbf4572db430fefeea3374286baf1)
==========

to

==========
M57: Add UseCounter for ServiceWorkerNavigationPreload.

We have a UMA already (ServiceWorker.NavigationPreload.HeaderSize) which gives
the total count, but it's hard to compute the % of page views without a
UseCounter.

BUG=649558

Review-Url: https://codereview.chromium.org/2659043002
Cr-Commit-Position: refs/heads/master@{#448529}
(cherry picked from commit d688b918acfbbf4572db430fefeea3374286baf1)

Review-Url: https://codereview.chromium.org/2684933011 .
Cr-Commit-Position: refs/branch-heads/2987@{#429}
Cr-Branched-From:
ad51088c0e8776e8dcd963dbe752c4035ba6dab6-refs/heads/master@{#444943}
Committed:
https://chromium.googlesource.com/chromium/src/+/c3c0fd9fbf6ae5659395b941b80d...
==========

[falken, 2017-02-10 05:32:44]

Committed patchset #1 (id:1) manually as
c3c0fd9fbf6ae5659395b941b80dc142e5ff83b9.