Remove hardcoded constant preprocessor conditions about canvas acceleration

In RenderLayerCompositor, a preprocessor constant was defined to
determine whether we should use accelerated canvas for small canvases.
This value is hardcoded as always true.  This patch removes the value
and simplifies the associated logic.

Review URL: https://codereview.chromium.org/54903005

git-svn-id: svn://svn.chromium.org/blink/trunk@161161 bbb929c8-8fbe-4397-9dbb-9b2b20218538