Don't use return value of compositeAndReadbackAsync so it can be void.

content/shell/renderer/test_runner/WebTestProxy.cpp

Index: content/shell/renderer/test_runner/WebTestProxy.cpp
diff --git a/content/shell/renderer/test_runner/WebTestProxy.cpp b/content/shell/renderer/test_runner/WebTestProxy.cpp
index e35e4de06d4f779f23635754135f0334d8ee69cd..8a30f274bf10374b0f32cc2b27bb2f27ec9f7007 100644
--- a/content/shell/renderer/test_runner/WebTestProxy.cpp
+++ b/content/shell/renderer/test_runner/WebTestProxy.cpp
@@ -488,24 +488,30 @@ void WebTestProxyBase::didCompositeAndReadback(const SkBitmap& bitmap) {
                bitmap.info().fHeight);
   SkCanvas canvas(bitmap);
   DrawSelectionRect(&canvas);
+  DCHECK(!m_compositeAndReadbackCallback.is_null());
   base::ResetAndReturn(&m_compositeAndReadbackCallback).Run(bitmap);
 }
 
 void WebTestProxyBase::CapturePixelsAsync(
     base::Callback<void(const SkBitmap&)> callback) {
-  m_compositeAndReadbackCallback = callback;
   TRACE_EVENT0("shell", "WebTestProxyBase::CapturePixelsAsync");
 
   // Do a layout here because it might leave compositing mode! x.x
   // TODO(danakj): Remove this when we have kForceCompositingMode everywhere.
   webWidget()->layout();
 
-  if (!webWidget()->compositeAndReadbackAsync(this)) {
+  if (!webWidget()->isAcceleratedCompositingActive()) {
     TRACE_EVENT0("shell",
                  "WebTestProxyBase::CapturePixelsAsync "
-                 "compositeAndReadbackAsync failed");
-    didCompositeAndReadback(SkBitmap());
+                 "isAcceleratedCompositingActive false");
+    callback.Run(SkBitmap());
+    return;
   }
+
+  DCHECK(!callback.is_null());
+  DCHECK(m_compositeAndReadbackCallback.is_null());
+  m_compositeAndReadbackCallback = callback;
+  webWidget()->compositeAndReadbackAsync(this);
 }
 
 void WebTestProxyBase::setLogConsoleOutput(bool enabled)