Move clipboard helpers from core/platform/chromium into platform.

To make the move easier (and clean up the code), I've removed
currentPasteboardBuffer() in favor of simply using Pasteboard::buffer().
The old code was silly anyway since it resulted in a conversion from
an enum value to a bool and then a bool back to an enum value.

BUG=297477
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/25560005

git-svn-id: svn://svn.chromium.org/blink/trunk@158767 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Web Animations CSS: Split AnimatableNumber into AnimatableDouble and AnimatableLength

AnimatableNumber currently does too many things. Splitting the class up
will allow us to subclass number functionality and make the overall
implementation cleaner.

This is a reland of http://crrev.com/25082007 after a change
conflict.

TBR=steveblock
NOTRY=TRUE
BUG=257591

Review URL: https://codereview.chromium.org/25678003

git-svn-id: svn://svn.chromium.org/blink/trunk@158768 bbb929c8-8fbe-4397-9dbb-9b2b20218538