Add plumbing for solid color scrollbar setting in Blink.

This CL adds the necessary plumbing in Blink to support the solid-color scrollbar
setting. It also adds an interface in WebCompositorSupport to allow Blink to
explicilty create either painted- or solid-color- scrollbars.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/22926020

git-svn-id: svn://svn.chromium.org/blink/trunk@156719 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add 8bit Content-Transfer-Encoding support for reading MHTML files

R=abarth@chromium.org
BUG=274232

Review URL: https://codereview.chromium.org/22292008

git-svn-id: svn://svn.chromium.org/blink/trunk@156720 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix interaction between ASAN and PartitionAlloc.

We now adhere to the expected contract of crashing upon allocation failure.

BUG=271306

Review URL: https://chromiumcodereview.appspot.com/23304017

git-svn-id: svn://svn.chromium.org/blink/trunk@156721 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Animated WebP: Optimize decoding in case of seeking

In case of WebP, it can be inferred whether an animation frame contains
alpha without decoding the frame.
WebP also has a mode where a frame overwrites the corresponding
rectangle on the previous frame.
These two were added to Chromium here:
https://codereview.chromium.org/22802020/

So, we optimize findRequiredPreviousFrame() for cases when a frame is
known to be opaque or it overwrites pixels on the previous frame.

BUG=229641

Review URL: https://chromiumcodereview.appspot.com/23068027

git-svn-id: svn://svn.chromium.org/blink/trunk@156723 bbb929c8-8fbe-4397-9dbb-9b2b20218538