Adding UKM and UMA tracking main frame task breakdown

The paint team has no insight into what proportion of the
Blink complete main frame time is spent in Blink lifecycle
operations vs. script required updates vs. other tasks.
This CL adds metrics that will allow for identifying the
proprtion of main frame time taken by various operations.

Bug: 869966
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Idc73dc6efd1aef5e65df647bc48de84384dba0c8
Reviewed-on: https://chromium-review.googlesource.com/1179995
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Robert Kaplow (sloooow) <rkaplow@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: vmpstr <vmpstr@chromium.org>
Commit-Queue: Stephen Chenney <schenney@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594114}

• cc/test/layer_tree_test.cc[diff]
• cc/test/stub_layer_tree_host_client.h[diff]
• cc/trees/layer_tree_host.cc[diff]
• cc/trees/layer_tree_host.h[diff]
• cc/trees/layer_tree_host_client.h[diff]
• cc/trees/proxy_main.cc[diff]
• content/browser/renderer_host/compositor_impl_android.h[diff]
• content/renderer/gpu/layer_tree_view.cc[diff]
• content/renderer/gpu/layer_tree_view.h[diff]
• content/renderer/gpu/layer_tree_view_delegate.h[diff]
• content/renderer/render_widget.cc[diff]
• content/renderer/render_widget.h[diff]
• content/test/stub_layer_tree_view_delegate.h[diff]
• third_party/blink/public/web/web_widget.h[diff]
• third_party/blink/renderer/core/exported/web_view_impl.cc[diff]
• third_party/blink/renderer/core/exported/web_view_impl.h[diff]
• third_party/blink/renderer/core/frame/BUILD.gn[diff]
• third_party/blink/renderer/core/frame/local_frame_ukm_aggregator.cc[Added - diff]
• third_party/blink/renderer/core/frame/local_frame_ukm_aggregator.h[Added - diff]
• third_party/blink/renderer/core/frame/local_frame_view.cc[diff]
• third_party/blink/renderer/core/frame/local_frame_view.h[diff]
• third_party/blink/renderer/core/frame/web_view_frame_widget.cc[diff]
• third_party/blink/renderer/core/frame/web_view_frame_widget.h[diff]
• third_party/blink/renderer/platform/BUILD.gn[diff]
• tools/metrics/histograms/histograms.xml[diff]
• tools/metrics/ukm/ukm.xml[diff]
• ui/compositor/compositor.h[diff]