Thorough tests for saturatedAdd and mulDiv255Round.

BUG=skia:3951

Committed: https://skia.googlesource.com/skia/+/ce9d11189a5924b47c3629063b72bae9d466c2c7

CQ_EXTRA_TRYBOTS=client.skia.android:Test-Android-GCC-Nexus5-CPU-NEON-Arm7-Release-Trybot

Review URL: https://codereview.chromium.org/1184113003
2 files changed