De-flaking some tests in virtual/gpu/fast/canvas

LayoutTests/fast/canvas/resources/asyncpaintguard.js

Index: LayoutTests/fast/canvas/resources/asyncpaintguard.js
diff --git a/LayoutTests/fast/canvas/resources/asyncpaintguard.js b/LayoutTests/fast/canvas/resources/asyncpaintguard.js
new file mode 100644
index 0000000000000000000000000000000000000000..dea5671973a61ada24286d4f2f1760a6052a2a15
--- /dev/null
+++ b/LayoutTests/fast/canvas/resources/asyncpaintguard.js
@@ -0,0 +1,13 @@
+// In tests that are not explicitly async, include this file to ensure
+// paint completion before the layout test pixel results are captured.
+
+if (window.testRunner)
+    testRunner.waitUntilDone();
+
+function finishTest() {
+    if (window.testRunner) {
+        testRunner.layoutAndPaintAsyncThen(function () { testRunner.notifyDone(); });
+    }
+}
+
+window.addEventListener("load", finishTest, false);