Roll external/skia 620b7e9b0..c726ca637 (3 commits)

https://skia.googlesource.com/skia.git/+log/620b7e9b0..c726ca637

2019-07-11 halcanary@google.com experimental/tools: set-change-id-hook
2019-07-11 robertphillips@google.com Alter approximate scratch texture binning after 1024
2019-07-11 scroggo@google.com Revert "Update Wuffs version"

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: I971e2afe5379b4f05ff5573c36a18c77563c80d6
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.