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Previously the pin button was 8dip from the edge of the view. The spec
calls for 4dip.

This CL also reduces scope of screen capture related constants.h

Bug: 1165348
Change-Id: I585b332e82735ecad2dcc2707928a39a2ebbc76c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2624030Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#842642}