Roll external/skia 831a29341..e71641479 (3 commits)

https://skia.googlesource.com/skia.git/+log/831a29341..e71641479

2019-04-15 reed@google.com removed localmatrix getter
2019-04-15 brianosman@google.com Extend --writeShaders to output SPIR-V
2019-04-15 egdaniel@google.com Add callback to flush for knowing when gpu is finished work.

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Change-Id: I00429abd3bf4cd6ae02e500acfc90a31de6da2da
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diff --git a/Android.bp b/Android.bp
index 5e0185d..b7eb528 100644
--- a/Android.bp
+++ b/Android.bp
@@ -1586,6 +1586,7 @@
         "tests/GrCCPRTest.cpp",
         "tests/GrContextAbandonTest.cpp",
         "tests/GrContextFactoryTest.cpp",
+        "tests/GrFinishedFlushTest.cpp",
         "tests/GrGLExtensionsTest.cpp",
         "tests/GrMemoryPoolTest.cpp",
         "tests/GrMeshTest.cpp",