cc: Fix PostiveRatio crash by checking raster scale

cc/picture_layer_impl.cc

Index: cc/picture_layer_impl.cc
diff --git a/cc/picture_layer_impl.cc b/cc/picture_layer_impl.cc
index bb608e84e00d27661ac0bb86599db4f79981bbca..1af6e686db1d85c56ae6195651e1455c3944c5f3 100644
--- a/cc/picture_layer_impl.cc
+++ b/cc/picture_layer_impl.cc
@@ -570,7 +570,7 @@ void PictureLayerImpl::ManageTilings(bool animating_transform_to_screen) {
     raster_source_scale_was_animating_ = animating_transform_to_screen;
   }
 
-  if (is_active_layer && is_pinching) {
+  if (is_active_layer && is_pinching && raster_page_scale_) {

[danakj, 2013/02/11 23:11:35]

I guess all of these if()s could be guarded by (...&& !change_target_tiling) to
deal with this, and ensure only the last positive case happens.

But I guess this way is also more ordering-independent for better or worse.

[enne (OOO), 2013/02/12 02:18:20]

Or put it all in a helper function? Anyway, I'm just trying to fix this crash
for now, so am going to land this as-is.

     // If the page scale diverges too far during pinch, change raster target to
     // the current page scale.
     float ratio = PositiveRatio(ideal_page_scale_, raster_page_scale_);