Implement SSE2-based implementations of the morphology filters (dilate & erode)....

Implement SSE2-based implementations of the morphology filters (dilate & erode). This gives a 3-5X speedup over the naive implementation, and also mitigates a timing-based security attack in Chrome (https://code.google.com/p/chromium/issues/detail?id=251711).

NOTE: this will require a corresponding GYP change on the Skia roll into Chrome: https://codereview.chromium.org/52453004/

R=mtklein@google.com, reed@google.com

Review URL: https://codereview.chromium.org/52603004

git-svn-id: http://skia.googlecode.com/svn/trunk@12038 2bbb7eff-a529-9590-31e7-b0007b416f81