Expect YUV to RGB to match exactly for C and SIMD

Unit tests to convert YUV to RGB previously allowed some error
for differences in implementation between C and SIMD versions,
and there were differences with Intel SSE2 vs ARM NEON.
Now those differences are resolved, the unittest can expect
exactness.

Bug: libyuv:447
Test: LibYUVConvertTest.I420ToARGB_Any
Change-Id: I00fe342523deba9420eb71812d09a8e89dc641a0
Reviewed-on: https://chromium-review.googlesource.com/946563Reviewed-by: Frank Barchard <fbarchard@chromium.org>
Commit-Queue: Frank Barchard <fbarchard@chromium.org>