Deprecating label Proj-Mustash-Mus-WS in favor of Components.

One stat I propose adding as part of this is the number of client hit tests we do for each event, e.g.
0 means we didn't need to fall back to client hit testing
1 means the first client we asked was the event target
2 means the second ...
etc.

This should help inform how often we are hitting the slow path, and how many round trips the slow path requires, which will help inform the priority of other issues such as issue 842678 or improved targeting such as issue 842684.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/69763ca95f4b19782bca7614c073b25d5671aa53

commit 69763ca95f4b19782bca7614c073b25d5671aa53
Author: Ria Jiang <riajiang@chromium.org>
Date: Sun May 20 11:04:22 2018

Collect metric for the number of clients we asked in async targeting.

Record the number of clients we asked when doing async targeting. 0
means we didn't take async path at all; 1 means we found the target
when asking the first client; 2 means we found the target when asking
the second client; ... This should give us a better idea of how many
clients we usually ask in async path so we can try to optimize and
reduce latency in async path.

Note that we only report this UMA if Site Isolation is turned on and
we actually did explicit hit-testing for that event.

Bug:  732417 
Change-Id: I164a9f8c853181c4ab6896a5bfcf9c0806a3da7d
Reviewed-on: https://chromium-review.googlesource.com/1060286
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Ria Jiang <riajiang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560198}
[modify] https://crrev.com/69763ca95f4b19782bca7614c073b25d5671aa53/content/browser/renderer_host/render_widget_host_input_event_router.cc
[modify] https://crrev.com/69763ca95f4b19782bca7614c073b25d5671aa53/content/browser/renderer_host/render_widget_targeter.cc
[modify] https://crrev.com/69763ca95f4b19782bca7614c073b25d5671aa53/content/browser/renderer_host/render_widget_targeter.h
[modify] https://crrev.com/69763ca95f4b19782bca7614c073b25d5671aa53/tools/metrics/histograms/histograms.xml

Ria: this is done yes?