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The CL adds an event filter that transforms input event properties
in the extended desktop environment. It currently scales scroll/fling
events' offset values based on the following logic:

1. Linear scaling w.r.t. the device scale factor, which is 2x for
   highDPI displays.
2. 1.2x scaling for external displays, since they are usually larger
   than internal ones.

Both scalings can stack on each other, i.e. you can get as high as
a 2.4x boost if you have a high DPI external display.

Contributed by sheckylin@chromium.org

BUG=chromium:166392
TEST=Tested on link w/o external display.

Change-Id: Ie29e03fcf9f6e5ae1dd6231990103237c3129fd6


Review URL: https://chromiumcodereview.appspot.com/11743013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@176038 0039d316-1c4b-4281-b951-d872f2087c98