Implement Asynchronous image encoding for Canvas.toBlob

LayoutTests/fast/canvas/canvas-toBlob-toDataURL-race-imageEncoder-png-expected.txt

Index: LayoutTests/fast/canvas/canvas-toBlob-toDataURL-race-imageEncoder-png-expected.txt
diff --git a/LayoutTests/fast/canvas/canvas-toBlob-toDataURL-race-imageEncoder-png-expected.txt b/LayoutTests/fast/canvas/canvas-toBlob-toDataURL-race-imageEncoder-png-expected.txt
new file mode 100644
index 0000000000000000000000000000000000000000..bd1443def3d2d13e70eca26ffff523eff0b74d14
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+++ b/LayoutTests/fast/canvas/canvas-toBlob-toDataURL-race-imageEncoder-png-expected.txt
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+Verifies if PNG image encoding on main thread (toDataURL) conflicts with image encoding on async thread (toBlob)
+
+On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
+
+
+PASS successfullyParsed is true
+
+TEST COMPLETE
+PASS canvasCtxs[0].getImageData(0, 0, 2500, 1750).data[25] is non-zero.
+PASS canvasCtxs[0].getImageData(0, 0, 2500, 1750).data[25] is canvasCtxs[1].getImageData(0, 0, 2500, 1750).data[25]
+PASS canvasCtxs[1].getImageData(0, 0, 2500, 1750).data[25] is canvasCtxs[2].getImageData(0, 0, 2500, 1750).data[25]
+PASS canvasCtxs[2].getImageData(0, 0, 2500, 1750).data[25] is canvasCtxs[3].getImageData(0, 0, 2500, 1750).data[25]
+PASS canvasCtxs[3].getImageData(0, 0, 2500, 1750).data[25] is canvasCtxs[4].getImageData(0, 0, 2500, 1750).data[25]
+PASS canvasCtxs[4].getImageData(0, 0, 2500, 1750).data[25] is canvasCtxs[5].getImageData(0, 0, 2500, 1750).data[25]
+PASS canvasCtxs[5].getImageData(0, 0, 2500, 1750).data[25] is canvasCtxs[6].getImageData(0, 0, 2500, 1750).data[25]
+PASS canvasCtxs[6].getImageData(0, 0, 2500, 1750).data[25] is canvasCtxs[7].getImageData(0, 0, 2500, 1750).data[25]
+PASS canvasCtxs[7].getImageData(0, 0, 2500, 1750).data[25] is canvasCtxs[8].getImageData(0, 0, 2500, 1750).data[25]
+PASS canvasCtxs[8].getImageData(0, 0, 2500, 1750).data[25] is canvasCtxs[9].getImageData(0, 0, 2500, 1750).data[25]
+PASS canvasCtxs[9].getImageData(0, 0, 2500, 1750).data[25] is canvasCtxs[10].getImageData(0, 0, 2500, 1750).data[25]
+PASS canvasCtxs[10].getImageData(0, 0, 2500, 1750).data[25] is canvasCtxs[11].getImageData(0, 0, 2500, 1750).data[25]
+