Move GraphicsContextTest from web to core.

And rename the test suite to match what it now tests.

R=jamesr@chromium.org
BUG=263554

Review URL: https://chromiumcodereview.appspot.com/19647012

git-svn-id: svn://svn.chromium.org/blink/trunk@154796 bbb929c8-8fbe-4397-9dbb-9b2b20218538