Ensure subframe timings aren't included in FCP++ metrics.

This test ensures that no merging of FCP++ timings is done by the
PageLoadMetricsUpdateDispatcher.

This follows https://chromium-review.googlesource.com/c/chromium/src/+/1334944,
which introduced a bug that merged FCP++ timings from subframes. That CL
has since been reverted. This test ensures we won't regress again.

Change-Id: Idbafb732e16f044301cb3f2cb2ab014bae0d9089
Reviewed-on: https://chromium-review.googlesource.com/c/1354271
Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Commit-Queue: Bryan McQuade <bmcquade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616361}
5 files changed