cc: Make single-thread renderer compositor always synchronous.

content/browser/gpu/compositor_util.cc

Index: content/browser/gpu/compositor_util.cc
diff --git a/content/browser/gpu/compositor_util.cc b/content/browser/gpu/compositor_util.cc
index 47b2d7077512cea0a8f4028b8e18ba928ed8b8ef..a652c39dbd9c6bc62ecff1ff5dc7a4e15dfb3486 100644
--- a/content/browser/gpu/compositor_util.cc
+++ b/content/browser/gpu/compositor_util.cc
@@ -257,6 +257,14 @@ bool IsOneCopyUploadEnabled() {
 bool IsZeroCopyUploadEnabled() {
   const base::CommandLine& command_line =
       *base::CommandLine::ForCurrentProcess();
+  // Single-threaded mode in the renderer process (for layout tests) is
+  // synchronous, which depends on tiles being ready to draw when raster is
+  // complete.  Therefore, it must use one of zero copy, software raster, or
+  // GPU raster. So we force zero-copy on for the case where software/GPU raster
+  // is not used.
+  // TODO(reveman): One-copy can work with sync compositing: crbug.com/490295.
+  if (command_line.HasSwitch(switches::kDisableThreadedCompositing))
+    return true;
   return command_line.HasSwitch(switches::kEnableZeroCopy);
 }