Previously, the broadcast step would not get run in case the main()
function returned before reaching its end. Now the broadcast step is
put in a separate function that wraps main() if needed, so that it
gets run even if the main() function in the original shader source
returns in the middle.

DrawBuffersTest is refactored to use ANGLETest::drawQuad() instead of
calling glDrawArrays directly.

BUG=angleproject:2109
TEST=WebGL conformance tests, angle_end2end_tests

Change-Id: Id5f05094e816df03bc9c8ca62b60de914072682c
Reviewed-on: https://chromium-review.googlesource.com/574597Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>