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As new style notification is launched in M62, and
IsNewStyleNotificationEnabled() returns true by default on Chrome OS,
we no longer have to check IsNewStyleNotificaitonEnabled().

This CL makes following changes.
- Remove IsNewStyleNotificationEnabled checks under
  chrome/browser/chromeos. (*.cc)
- Remove UI strings that are no longer used. (*.grdp)
- Remove icons that are no longer used. (*.png, *.icon)
Icons that are not removed are referenced in somewhere.

BUG=757703

Change-Id: I46e52295115c3e0a619d1eeefb02321034ce5117
Reviewed-on: https://chromium-review.googlesource.com/802601Reviewed-by: Evan Stade <estade@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522004}