Roll external/skia c726ca637..7825d4983 (5 commits)

https://skia.googlesource.com/skia.git/+log/c726ca637..7825d4983

2019-07-11 benjaminwagner@google.com Add comparison of rendered SkPictures to Serialization unit test
2019-07-11 egdaniel@google.com Add kRGBA_8888_SRGB GrColorType and remove GrSRGBEncoded.
2019-07-11 halcanary@google.com Sample/Camera: make it work without /skimages
2019-07-11 mtklein@google.com rename to bt709_luminance_or_luma_to_alpha
2019-07-11 jlavrova@google.com Fix for Windows specific compiler issues (constexpr).

The AutoRoll server is located here: https://skia-autoroll.corp.goog/r/android-master-autoroll

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff, who should
be CC'd on the roll, and stop the roller if necessary.

Test: Presubmit checks will test this change.
Change-Id: I41a7c5fd67e548b2bc1f0f7b41362e1c92180b36
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.