cc: Fix PostiveRatio crash by checking raster scale

The first time through ManageTilings, raster scale is zero, so you can't
use it to find a ratio.  This DCHECKS because the ratio is infinite.
If the raster scale is zero, we already create new tilings, so skipping
this check will just avoid the DCHECK.

R=reveman@chromium.org
BUG=174236


Review URL: https://chromiumcodereview.appspot.com/12209103

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@181857 0039d316-1c4b-4281-b951-d872f2087c98

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_) {
     // 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_);