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- Enables new implementation of touch support for screen magnification
  by default.
- Removes the old implementation of touch support for screen
  magnification.

      around with two fingers scroll without any flag.
      Pass ash_unittests:MagnificationControllerTest

Bug: 670526
Test: Confirm that you can change zoom level with pinch gesture and pan
Change-Id: Icf25edb2bceae28323075f857c3e74c2b4526dcc
Reviewed-on: https://chromium-review.googlesource.com/903306Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Yuki Awano <yawano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534861}