Roll src/third_party/skia a76fad2d494a..731ba99a8581 (20 commits) (#8686)

https://skia.googlesource.com/skia.git/%2Blog/a76fad2d494a..731ba99a8581

git log a76fad2d494af8bde8c3fb34a2de2b359beebab1..731ba99a8581cb81978e8217d210309880558b09 --date=short --no-merges --format=%ad %ae %s
2019-04-22 mtklein@google.com deshard NP and N7 CPU test bots
2019-04-22 mtklein@google.com Revert "simplify and centralize SK_API"
2019-04-22 robertphillips@google.com Add explicit GrSurfaceProxy flag to skip explicit resource allocation (take 2)
2019-04-22 mtklein@google.com remove duplicate SkBase64.h
2019-04-22 mtklein@google.com rm src/compute
2019-04-22 borenet@google.com [docs] Update documentation for branching process
2019-04-11 mtklein@google.com simplify and centralize SK_API
2019-04-22 egdaniel@google.com Fully add back old GrContext flush api.
2019-04-15 mtklein@google.com add constexpr constructor to SkMatrix
2019-04-22 egdaniel@google.com Update comment on vulkan allocator.
2019-04-22 egdaniel@google.com Add back deprecated GrCotnext flush API till clients get fixed.
2019-04-19 brianosman@google.com Add SkSL viewing/editing to viewer
2019-04-22 mtklein@google.com Revert "Add explicit GrSurfaceProxy flag to skip explicit resource allocation"
2019-04-22 skia-recreate-skps@skia-swarming-bots.iam.gserviceaccount.com Update go_deps asset
2019-04-22 skia-autoroll@skia-public.iam.gserviceaccount.com Roll third_party/externals/angle2 145ec7fa7c81..c467f7b54188 (4 commits)
2019-04-22 skia-autoroll@skia-public.iam.gserviceaccount.com Roll third_party/externals/swiftshader a16238df3f28..bcc71a322953 (7 commits)
2019-04-22 skia-autoroll@skia-public.iam.gserviceaccount.com Roll ../src a55917cb9824..3ed0f75776bb (344 commits)
2019-04-21 reed@google.com use portable typeface for new gm
2019-04-21 skia-recreate-skps@skia-swarming-bots.iam.gserviceaccount.com Update SKP version
2019-04-21 rmistry@google.com Skip __init__.py in webpages_playback.py

The AutoRoll server is located here: https://autoroll.skia.org/r/skia-flutter-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 (mtklein@google.com), and stop
the roller if necessary.

DEPS

@@ -23,7 +23,7 @@ vars = {
   'fuchsia_git': 'https://fuchsia.googlesource.com',
   'github_git': 'https://github.com',
   'skia_git': 'https://skia.googlesource.com',
-  'skia_revision': 'a76fad2d494af8bde8c3fb34a2de2b359beebab1',
+  'skia_revision': '731ba99a8581cb81978e8217d210309880558b09',
 
   # When updating the Dart revision, ensure that all entries that are
   # dependencies of Dart are also updated to match the entries in the