Roll external/skia 6613cc518..c2a4101e4 (7 commits)

https://skia.googlesource.com/skia.git/+log/6613cc518..c2a4101e4

2017-12-19 update-skps@skia.org Update SKP version
2017-12-19 angle-deps-roller@chromium.org Roll skia/third_party/externals/angle2/ f2afccca7..ae4dbf32b (1 commit)
2017-12-19 scroggo@google.com Consider overflow in is_orientation_marker
2017-12-19 kjlubick@google.com Make sure we get the LITTLE cores out of powersave
2017-12-19 caryclark@skia.org disable large rect dash temporarily
2017-12-19 ethannicholas@google.com fixed a couple of SkSL ushort issues
2017-12-19 caryclark@skia.org efficiently dash very large rectangles and very long lines

The AutoRoll server is located here: https://android-roll.skia.org

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: I00e86401eb5c9303afe8469ffc072139ffbec06b
Exempt-From-Owner-Approval: The autoroll bot does not require owner approval.