Output layout qualifiers for vertex shader attributes and fragment
shader outputs. This applies both to ESSL output and GLSL output. If
the source shader is in ESSL1, the code has no effect since there's
no layout information in the AST.

BUG=525930
TEST=angle_unittests, angle_end2end_tests

Change-Id: I48b3aa56116c15d11599b030eed4c45be2c8fc7e
Reviewed-on: https://chromium-review.googlesource.com/304550
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>