This will enable more parts of Chromium to use PA, and will make it easier for things outside of Chromium (e.g. PDFium) to use PA.

This change is intended to be merely a move, and should not have significant performance impact.

This moves only the core of PA; higher API layers that Blink uses to call into PA remain in Blink.

BUG=632441
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2518253002
Cr-Commit-Position: refs/heads/master@{#438471}