Roll external/skia c6611c0de..fec9b902a (2 commits)

https://skia.googlesource.com/skia.git/+log/c6611c0de..fec9b902a

2019-06-03 mtklein@google.com add unit scaling mode to benchmark/nanobench
2019-06-03 borenet@google.com [infra] Stop isolating the Android NDK

The AutoRoll server is located here: https://autoroll-internal.skia.org/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: I438ec428f1c21e9b52fc784329831b606fd4eeca
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.