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Disable animations to get TextfieldTest.DragToSelect to pass.
AuraTestHelper disables animations, so they were already disabled on non-Mac.
This change creates ViewsTestHelperMac that disables animation the same way
as ViewsTestHelperAura does.

BUG=378134

Review URL: https://codereview.chromium.org/378663002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282385 0039d316-1c4b-4281-b951-d872f2087c98