Enable needed extensions in generated GLSL shaders and call them when possible. reland: use the defined preprocessor function. BUG=angleproject:1044 Change-Id: I6171708a2d55ec085ba2c598a1a863779842da6c Reviewed-on: https://chromium-review.googlesource.com/311270 Tryjob-Request: Geoff Lang <geofflang@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>