Clean up incorrect getter function names in WebGL related classes in blink

There are no logic changes, only function name changes.

BUG=293775
TEST=tree

Review URL: https://chromiumcodereview.appspot.com/24209002

git-svn-id: svn://svn.chromium.org/blink/trunk@157942 bbb929c8-8fbe-4397-9dbb-9b2b20218538