Move Widget.cpp/.h to platform/

BUG=297477

Review URL: https://codereview.chromium.org/27122002

git-svn-id: svn://svn.chromium.org/blink/trunk@159538 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Revert "Make composited scrolling codepaths co-operate."

This reverts commit 0a2788a1c5a19e5a7d11f4d21f9e6c38d89217de.

>    Since we don’t have a functional layer-squashing solution
>    yet, it makes sense to leave in the logic for the stacking
>    container concept for the short term, and use it whenever
>    possible. If a layer cannot become a stacking container,
>    then we fall back to using the new codepath that’s being set
>    up in all the above patches.
>
>    This is blink-side patch 6 of https://docs.google.com/a/chromium.org/document/d/1Ln57_v6_4RmuuvtWnpsfBOUeDFrPsYNFW13dwuQ8UD8.
>
>    BUG=254111
>
>    Review URL: https://codereview.chromium.org/20103002

TBR=hartmanng@chromium.org
BUG=306566

Review URL: https://codereview.chromium.org/27128002

git-svn-id: svn://svn.chromium.org/blink/trunk@159539 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Move Region.cpp/.h to platform/

BUG=297477

Review URL: https://codereview.chromium.org/27123002

git-svn-id: svn://svn.chromium.org/blink/trunk@159541 bbb929c8-8fbe-4397-9dbb-9b2b20218538