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When pinch zooming with a Mac trackpad, we generate synthetic ctrl-wheels
so that JS can handle the gesture. We now use these wheel events in the
PDF viewer's gesture detector to create pinch events for the viewer's
pinch zoom mechanism, as we currently do for pinches from touch events.

We also use this for non-synthetic ctrl-wheels. This allows us to have
zooming with the mouse wheel anchor the zoom around the mouse position
instead of the scroll position.

Bug: 715662, 715670
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ia672a266cdd69c3e87d1451548c7470176ed8fd6
Reviewed-on: https://chromium-review.googlesource.com/730648Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Kevin McNee <mcnee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516034}