Record CachedMetrics in renderers

While reworking ChromiumAndroidLinker.{BrowserStates,RendererStates} I
noticed that "RelinkerFallbackCount" is not recorded in renderers. This
is also observable on the UMA dashboard: for my time interval I see
these sample counts:
ChromiumAndroidLinker.RendererLoadTime: N samples
ChromiumAndroidLinker.BrowserLoadTime: (0.8 * N) samples
ChromiumAndroidLinker.RelinkerFallbackCount: only about N samples, but
    expected to be closer to 2 * N.

I *think* that "RelinkerFallbackCount" is the only renderer-side
histogram that is recorded with CachedMetrics currently, and in my rough
testing this change helps recording more samples. Removing this
histogram to re-introduce it as part of another metric.

Tests TBD as part of another change where the new histogram is
introduced.

There is a way to call commitCachedMetrics() for the browser process as
well, but I'd like to postpone it now because it may significantly
affect a number of metrics (see https://crbug.com/986796).

Bug: 981599
Change-Id: I69799f96278fe5c6ea8661055aa153a8b7f4fb46
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1713535
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680001}