cc: Add a UMA to measure raster scheduling delay

This CL adds the Renderer4.{Browser,Renderer}.RasterTaskSchedulingDelay
UMAs which measure the wall time elapsed between creating a raster task
in the client and being ready to issue raster work to the driver in the
GPU process.

The beginning timestamp is obtained by recording the creation time of a
RasterBufferImpl in the renderer (which should practically coincide with
the creation of the raster tasks). The end timestamp is obtained using a
GL_COMMANDS_ISSUED_TIMESTAMP_CHROMIUM command buffer query.

The motivation is to make sure that hardware accelerated image decodes
do not make this part of the rendering pipeline regress. Therefore,
these UMAs are only computed and recorded in Chrome OS and when OOP-R is
enabled.

Bug: 995155
Test: coverage for the new UMA.
Change-Id: I51d4b2d87c07577ca5f710d94a15dac2e4312ce9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1753583
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689975}

• cc/raster/gpu_raster_buffer_provider.cc[diff]
• cc/raster/gpu_raster_buffer_provider.h[diff]
• cc/raster/raster_buffer_provider_unittest.cc[diff]
• components/viz/test/test_gles2_interface.cc[diff]
• tools/metrics/histograms/histograms.xml[diff]