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Collects histograms from child processes (GPU, renderer, etc.) before
sending metrics over the fuchsia.legacymetrics.MetricsRecorder
interface.

Implement upload batching, to break up large report calls into
smaller chunks that fit within the FIDL message limit.

Make the AdditionalMetricsCallback interface asynchronous, so that
async data collection may take place.

Bug: 1060768
Change-Id: I4d4ae5f168ecbeb65a8d9745e940fb05f68e2399
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2147683
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759033}