Simplify GraphicsContext::couldUseLCDRenderedText()

After https://codereview.chromium.org/719253002, Skia disables LCD text
for non-opaque save-layers automatically.

Also remove a couple of single-use getters.

R=reed@google.com,jbroman@chromium.org,senorblanco@chromium.org

Review URL: https://codereview.chromium.org/724063004

git-svn-id: svn://svn.chromium.org/blink/trunk@185342 bbb929c8-8fbe-4397-9dbb-9b2b20218538