SyncPointManager never returns 0.

When the next sync point overflow occurs, it returns 1, not 0.

BUG=365904, 373452

Review URL: https://codereview.chromium.org/288153004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@270997 0039d316-1c4b-4281-b951-d872f2087c98

content/common/gpu/sync_point_manager.cc

@@ -26,6 +26,9 @@ SyncPointManager::~SyncPointManager() {
 uint32 SyncPointManager::GenerateSyncPoint() {
   base::AutoLock lock(lock_);
   uint32 sync_point = next_sync_point_++;
+  // When an integer overflow occurs, don't return 0.
+  if (!sync_point)
+    sync_point = next_sync_point_++;
 
   // Note: wrapping would take days for a buggy/compromized renderer that would
   // insert sync points in a loop, but if that were to happen, better explicitly

content/common/gpu/sync_point_manager.h

@@ -22,7 +22,7 @@ class SyncPointManager : public base::RefCountedThreadSafe<SyncPointManager> {
   SyncPointManager();
 
   // Generates a sync point, returning its ID. This can me called on any thread.
-  // IDs start at 1.
+  // IDs start at a random number. Never return 0.
   uint32 GenerateSyncPoint();
 
   // Retires a sync point. This will call all the registered callbacks for this