Re-land: cc: Fail more visibly when sync queries are not working correctly.

cc/output/gl_renderer.cc

Index: cc/output/gl_renderer.cc
diff --git a/cc/output/gl_renderer.cc b/cc/output/gl_renderer.cc
index 8e60f944d4f6f23cf033f15f2f7c619473fe99cf..df3ddfd3b9e865906172309006e8e45d91ded448 100644
--- a/cc/output/gl_renderer.cc
+++ b/cc/output/gl_renderer.cc
@@ -175,6 +175,10 @@ SamplerType SamplerTypeFromTextureTarget(GLenum target) {
 // determine when anti-aliasing is unnecessary.
 const float kAntiAliasingEpsilon = 1.0f / 1024.0f;
 
+// Block or crash if the number of pending sync queries reach this high as
+// something is seriously wrong on the service side if this happens.
+const size_t kMaxPendingSyncQueries = 16;
+
 }  // anonymous namespace
 
 class GLRenderer::ScopedUseGrContext {
@@ -248,6 +252,11 @@ class GLRenderer::SyncQuery {
     return !available;
   }
 
+  void Wait() {
+    unsigned result = 0;
+    gl_->GetQueryObjectuivEXT(query_id_, GL_QUERY_RESULT_EXT, &result);
+  }
+
  private:
   class Fence : public ResourceProvider::Fence {
    public:
@@ -441,6 +450,15 @@ void GLRenderer::BeginDrawingFrame(DrawingFrame* frame) {
 
   scoped_refptr<ResourceProvider::Fence> read_lock_fence;
   if (use_sync_query_) {
+    // Block until oldest sync query has passed if the number of pending queries
+    // ever reach kMaxPendingSyncQueries.
+    if (pending_sync_queries_.size() >= kMaxPendingSyncQueries) {
+      LOG(ERROR) << "Reached limit of pending sync queries.";
+
+      pending_sync_queries_.front()->Wait();
+      DCHECK(!pending_sync_queries_.front()->IsPending());
+    }
+
     while (!pending_sync_queries_.empty()) {
       if (pending_sync_queries_.front()->IsPending())
         break;