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This CL removes references to IsSystemTrayUnifiedEnabled in
chrome/browser/. tray_accessibility_browsertest and first_run_controller
are intentionally left as the first one will be moved to ash/,
and the second one will be updated in a separate CL.

This is the part 2 of removing old System Tray code.

(1) Removing the flag
(2) Removing references to IsSystemTrayUnifiedEnabled()
(3) Removing classes that are only used in old System Tray

I'll try to keep the number of CLs as small as possible to make the
history clean, but there can be multiple CLs for both part 2 and part 3
because some of them requires nontrivial refactoring to remove the
dependency to the old code.

TEST=trybot pass
BUG=898419

Change-Id: Ib1d046bae25a61354bba5dd8d0ce62ba938c015f
Reviewed-on: https://chromium-review.googlesource.com/c/1301054Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603405}