cc: Make single-thread renderer compositor always synchronous.

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 248e744c417d87cb610e4d24bcb22d861d3b6cb0..aca271f256ba63d29cf948cccb8fc7dcfbba4df0 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2118,7 +2118,7 @@ void LayerTreeHostImpl::CreateResourceAndTileTaskWorkerPool(
         image_target == GL_TEXTURE_EXTERNAL_OES,
         context_provider->ContextCapabilities().gpu.egl_image_external);
 
-    if (settings_.use_zero_copy || IsSynchronousSingleThreaded()) {
+    if (settings_.use_zero_copy) {
       *resource_pool =
           ResourcePool::Create(resource_provider_.get(), image_target);
 
@@ -2128,6 +2128,11 @@ void LayerTreeHostImpl::CreateResourceAndTileTaskWorkerPool(
     }
 
     if (settings_.use_one_copy) {
+      // Synchronous single-threaded mode depends on tiles being ready to
+      // draw when raster is complete.  Therefore, it must use one of zero

[reveman, 2015/05/20 19:13:32]

fyi, tiles are ready to draw when raster is complete in the one-copy case too.
the problem is that one-copy currently rely on being able to communicate with
the compositor thread from worker threads. this problem will go away when
one-copy uses a separate context just like gpu raster. just filed
crbug.com/490295 for that. please add a TODO here and/or in
IsZeroCopyUploadEnabled().

[danakj, 2015/05/20 19:16:33]

Done.

+      // copy, software raster, or GPU raster.
+      DCHECK(!IsSynchronousSingleThreaded());
+
       // We need to create a staging resource pool when using copy rasterizer.
       *staging_resource_pool =
           ResourcePool::Create(resource_provider_.get(), image_target);