Add command line switch to force prefers-reduced-motion

This enables users who are on platforms where we do not yet support
prefers-reduced-motion (primarily Linux) or where the user does not have
control over the system settings to launch Chrome in the
prefers-reduced-motion mode.

Bug: 929310
Change-Id: I1e6c1175911b769198aeb3a4f0e00da3ed9f3f80
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1671668
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672344}
4 files changed