Remove unnecessary 'const's

Noticed here https://codereview.chromium.org/538333002/ .
Something's wrong between gn/gyp because this doesn't happen
in gyp, but remove these consts anyway as they're unnecessary.

R=thakis@chromium.org
BUG=335824

Review URL: https://codereview.chromium.org/551863004

git-svn-id: svn://svn.chromium.org/blink/trunk@181584 bbb929c8-8fbe-4397-9dbb-9b2b20218538