It's allowed to not have all code paths return a value in ESSL but the HLSL
compiler detects this and generates an error.  Work around this by adding
dummy return statements at the end of each function that doesn't have one.

TEST=deqp/data/gles2/shaders/functions.html

BUG=angleproject:1015
BUG=478572

Change-Id: I2913f90f0994d4caf25cc43b16b9fc4e9efb19a5
Reviewed-on: https://chromium-review.googlesource.com/362085Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>