CC/GPU: Add a soft limit to the compositor.

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 6829156565e085b73fe85fd9cbdc9942ca8303fe..cef1159589942fcbc5d5721211f3e457f6c25bc6 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -1171,11 +1171,19 @@ void LayerTreeHostImpl::UpdateTileManagerMemoryPolicy(
 
   // TODO(reveman): We should avoid keeping around unused resources if
   // possible. crbug.com/224475
-  global_tile_state_.memory_limit_in_bytes =
-      visible_ ?
-      policy.bytes_limit_when_visible : 0;
+  global_tile_state_.hard_memory_limit_in_bytes = 0;
+  global_tile_state_.soft_memory_limit_in_bytes = 0;
+  if (visible_ && policy.bytes_limit_when_visible > 0) {
+    global_tile_state_.hard_memory_limit_in_bytes =
+        policy.bytes_limit_when_visible;
+    global_tile_state_.soft_memory_limit_in_bytes =
+       global_tile_state_.hard_memory_limit_in_bytes *
+       settings_.max_memory_for_prepaint_percentage / 100;
+  }
+  // Unused limit is calculated from soft-limit, as hard-limit may
+  // be very high and shouldn't typically be exceeded.
   global_tile_state_.unused_memory_limit_in_bytes = static_cast<size_t>(
-      (static_cast<int64>(global_tile_state_.memory_limit_in_bytes) *
+      (static_cast<int64>(global_tile_state_.soft_memory_limit_in_bytes) *
        settings_.max_unused_resource_memory_percentage) / 100);
   global_tile_state_.memory_limit_policy =
       ManagedMemoryPolicy::PriorityCutoffToTileMemoryLimitPolicy(