If the GL_EXT_draw_buffers extension isn't explicitly enabled in the shader,
then gl_FragData is an array of size 1, not of size max_draw_buffers.

BUG=angleproject:1441
TEST=webgl2_conformance with --use-gl=angle

Change-Id: I2ead1457462bf1f396fda1f47022df6b54612e17
Reviewed-on: https://chromium-review.googlesource.com/362781Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Zhenyao Mo <zmo@chromium.org>