Remove unnecessary updateGraphicsContext calls.

These don't do anything, since all it does is update the fill color, which is
not even used in drawHighlightForText (it takes its own color argument).

After this, nearly all updateGraphicsContext calls correspond to a
paintTextWithShadows call, which opens some refactoring options.

Review URL: https://codereview.chromium.org/477143002

git-svn-id: svn://svn.chromium.org/blink/trunk@180376 bbb929c8-8fbe-4397-9dbb-9b2b20218538