make sure to_srgb maps 1 to 1

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Ubuntu-Clang-GCE-CPU-AVX2-x86_64-Release-SK_CPU_LIMIT_SSE2,Test-Ubuntu-Clang-GCE-CPU-AVX2-x86_64-Release-SK_CPU_LIMIT_SSE41,Test-Android-Clang-Nexus10-CPU-Exynos5250-arm-Release-Android,Test-Android-Clang-PixelC-CPU-TegraX1-arm64-Release-Android,Test-Android-Clang-Ci20-CPU-IngenicJZ4780-mipsel-Release-Android

BUG=skia:6678,skia:6683

Change-Id: I217084fa0a11ad661a8751f0c3b1cade5cc52473
Reviewed-on: https://skia-review.googlesource.com/17902
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
4 files changed