Use ContextProvider subclass to provide WGC3D pointers

Some cc::ContextProvider instances will also need to provide WebGraphicsContext3D*
interfaces to the underlying GL context, but not all. This adds a subclass of ContextProvider
in content:: to use for these users. The compositor itself doesn't need the WGC3D
accessor so it just uses the base class.

BUG=181120

Review URL: https://codereview.chromium.org/126093010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244383 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244383
[Telemetry] Some fixups in speed index calculation.

1. zIndex is case sensitive. Page contents were bleeding through on some pages.
2. With android screen capture compression artifacts, a slightly higher
   tolerance was needed.
3. The assert was wrong. We need to make sure the whole bounding box is filled.
   It doesn't matter what percent of the overall video is the bounding box.

BUG=323813

Review URL: https://codereview.chromium.org/127373003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244384 0039d316-1c4b-4281-b951-d872f2087c98
Clarify chrome.events as a namespace of common types.

Particualarly for apps documentation, a developer who first encounters
chrome.events may be slightly confused about all the documentation
without the context that it is all in support of other APIs.

BUG=325011
NOTRY=true

Review URL: https://codereview.chromium.org/135083002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244385 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244385
Tweak ifdefs for MessageLoopForUI::Add/RemoveObserver

This is necessitated by our building of base for tools. In particular
if I set OS_ANDROID and USE_AURA then the host is built with OS_LINUX
and USE_AURA, but none of the other flags we normally set if USE_AURA
is set for OS_LINUX.

And yes, I really need to spend time and refactor messageloop. It is
such a mess of ifdefs right now.

BUG=none
TEST=none
R=darin@chromium.org

Review URL: https://codereview.chromium.org/129873003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244386 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244386