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Currently, app windows (possibly containing webviews):
1) suppress all pinch events which also suppresses the
   synthetic wheel event, but not touchscreen touch events, and
2) allow "smart zoom" on Mac which can change the page scale even
   though other means of changing the page scale are suppressed.

As a result of (1), if we have, say, google maps in a webview, a
user can pinch zoom with a touchscreen, but not with a touchpad.

We now suppress gesture events to app windows based on whether they
cause a scale change. So now touchscreen and touchpad pinching are
consistent since the page has access to the events so that it may
implement custom pinch zoom behaviour, but unhandled events still
do not result in a scale change for the app window. Also, smart
zoom is suppressed to be consistent with other touchpad pinching.

Bug: 725970, 874132
Change-Id: I03dd2048002d69dc7c8a822fc727140c67d64706
Reviewed-on: https://chromium-review.googlesource.com/1174933Reviewed-by: Ben Wells <benwells@chromium.org>
Reviewed-by: James MacLean <wjmaclean@chromium.org>
Reviewed-by: Ehsan Karamad <ekaramad@chromium.org>
Commit-Queue: Kevin McNee <mcnee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585489}