Moving GraphcisContext and dependencies from core to platform.

The files in this move are all those, and only those, that must
be moved in order to move GraphicsContext without introducing layering
violations.

All font-related classes have been moved to fonts.

Image related classes have still to be moved to images.

Scroll related classes have been moved to scroll.

Some Skia classes have been moved to graphics/skia.

Some other code cleanup was done.

Some test files that rely on Platform features (current working
directory, feature enabled checking, and threading) have been moved
to web/tests. Some can be moved back when some of those Platform
test features are added back to blink_platform_unittests (patch
in preparation).

R=abarth@chromium.org, abarth, senorblanco
BUG=303332

Review URL: https://codereview.chromium.org/99103006

git-svn-id: svn://svn.chromium.org/blink/trunk@163315 bbb929c8-8fbe-4397-9dbb-9b2b20218538