TaskGraphRunner Group support

cc/raster/synchronous_task_graph_runner.cc

Index: cc/raster/synchronous_task_graph_runner.cc
diff --git a/cc/raster/synchronous_task_graph_runner.cc b/cc/raster/synchronous_task_graph_runner.cc
index ed05cfb5823127277020491b4b5fbb0d6e3abbd2..4c4c8ca673e7d79bfc74ff872747318167bc5056 100644
--- a/cc/raster/synchronous_task_graph_runner.cc
+++ b/cc/raster/synchronous_task_graph_runner.cc
@@ -7,6 +7,7 @@
 #include "base/threading/simple_thread.h"
 #include "base/threading/thread_restrictions.h"
 #include "base/trace_event/trace_event.h"
+#include "cc/raster/task_category.h"
 
 namespace cc {
 
@@ -64,7 +65,16 @@ void SynchronousTaskGraphRunner::RunUntilIdle() {
 void SynchronousTaskGraphRunner::RunTask() {
   TRACE_EVENT0("toplevel", "SynchronousTaskGraphRunner::RunTask");
 
-  auto prioritized_task = work_queue_.GetNextTaskToRun();
+  // Find the first category with any tasks to run. Categories used by this task
+  // graph runner are ordered by priority.
+  uint16_t category;
+  for (category = 0u; category < kNumTaskCategories; ++category) {

[reveman, 2015/12/10 16:49:57]

I'd like to avoid making the kNumTaskCategories assumption. See my other
comment.

[ericrk, 2015/12/14 19:40:57]

What do you think about this approach? It's a bit annoying that we have to do
find_if (as we can't just take the first one, as the list could be empty) - I
thought about trying to write some sort of iterator adapter to just show
non-empty elements, but seemed like more trouble than it was worth.

+    if (work_queue_.HasReadyToRunTasksForCategory(category))
+      break;
+  }
+  DCHECK(category < kNumTaskCategories);
+
+  auto prioritized_task = work_queue_.GetNextTaskToRun(category);
 
   Task* task = prioritized_task.task;
   task->WillRun();