Roll external/skia 6e637472ebf3..7dfd4bd3fce1 (2 commits)

https://skia.googlesource.com/skia.git/+log/6e637472ebf3..7dfd4bd3fce1

2019-09-01 skia-recreate-skps@skia-swarming-bots.iam.gserviceaccount.com Update SKP version
2019-09-01 skia-recreate-skps@skia-swarming-bots.iam.gserviceaccount.com Update Go deps

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://skia-autoroll.corp.goog/r/android-master-autoroll
Please CC scroggo@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

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

Test: Presubmit checks will test this change.
Change-Id: I3ac6790eb15c4640e78ca485763388065bad6129
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.