cc: Fix ideal_content_scale check in PictureImageLayerImpl

PictureImageLayerImpl ignores PictureLayerImpl::ideal_content_scale_ and
conditionally sets tile scale to 1.f. This makes the comparison in
PictureLayerImpl::AppendQuads to check for incomplete tiles incorrect.

Fix by overriding PictureImageLayerImpl::CalculateContentsScale to
always set ideal_content_scale_ to 1.f.

Fix for internal bug b/9103603.

TEST=PictureImageLayerImplTest.IgnoreIdealContentScale
BUG=

Review URL: https://codereview.chromium.org/26353002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227614 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227614