Return a transparent black ImageBitmap when mailbox is invalid

third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp

Index: third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp
diff --git a/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp b/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp
index ebed1a63bf97b77e8b8df2cf6e7183f5125193fd..6b92594436ae742bbde7ed2d3dfe4c1091119ee7 100644
--- a/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp
+++ b/third_party/WebKit/Source/modules/webgl/WebGLRenderingContextBase.cpp
@@ -642,7 +642,14 @@ ImageBitmap* WebGLRenderingContextBase::transferToImageBitmapBase()
         return nullptr;
     WebExternalTextureMailbox mailbox;
     drawingBuffer()->prepareMailbox(&mailbox, 0);
-    ImageBitmap* imageBitmap = ImageBitmap::create(mailbox);
+    ImageBitmap* imageBitmap;
+    // If the mailbox is invalid, return an transparent black ImageBitmap

[Ken Russell (switch to Gerrit), 2016/07/12 22:52:04]

Could you add a comment saying that the only situation this should happen is
when two or more calls to transferToImageBitmap are made back-to-back?

+    if (mailbox.textureSize.width == 0 && mailbox.textureSize.height == 0) {
+        sk_sp<SkSurface>surface = SkSurface::MakeRasterN32Premul(drawingBuffer()->size().width(), drawingBuffer()->size().height());
+        imageBitmap = ImageBitmap::create(StaticBitmapImage::create(fromSkSp(surface->makeImageSnapshot())));
+    } else {
+        imageBitmap = ImageBitmap::create(mailbox);
+    }
     // TODO(xidachen): Create a small pool of recycled textures from ImageBitmapRenderingContext's
     // transferFromImageBitmap, and try to use them in DrawingBuffer.
     return imageBitmap;