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With a chance of 1/256, we'd read past the end of kValidLiteralValues.
In practice, that read would return zeroes, which we'd write to
kTestLibData -- but zeroes aren't valid literal values, so that would
then make TestShader() crash when it tried to link the program.

Not sure if this is related to crbug.com/1051923, but it's in the
same area at least.

Add a components/metal_util:unit_tests target. Covering code is useful
even if the test doesn't verify a lot, because then the ASan bots can
find stuff like this for us. (...but in this case, asan doesn't find
the out-of-bounds read because we build with `-asan-globals=0` on
macOS because of https://crbug.com/352073 .
But the test also checks for non-0 and runs the code often enough
that the test consistently fails before the fix and consistently passes
with it.)

Drive-by no-op cleanups while here:
- Remove EXPORTs on enums, they don't have any effect
- Remove redundant is_ios block in components/BUILD.gn

Bug: 1147027,1051923,1085899
Cq-Include-Trybots: luci.chromium.try:mac_chromium_asan_rel_ng
Change-Id: Ia7f9735e450c70eb64b826a499dc60690714112e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2526161
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825730}