Fix a crash with the WebGL conformance test suite.

Undefined behavior caused by an overflowed enum was causing
the translator symbol table stack to not get popped correctly
when the compiler completed, causing dangling symbols who's storage
was reclaimed and reused when the allocation pool was popped.
The error occurred in the "shader-with-reserved-words" test, which
passes when the compiler fails due to the use of reserved words as
identifiers. As such, this test would pass even if the GPU process
crashes but further tests in the test suite would fail.

BUG=angle::785, 786

Change-Id: I365cb55f962f8dfe409f40532effeb10b8189432
Reviewed-on: https://chromium-review.googlesource.com/223093
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Nicolas Capens <capn@chromium.org>
Tested-by: Gus Fernandez <gusfernandez@chromium.org>