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The size of the frame data from a delegated source will not in general match up
exactly with the delegated layer's bounds. Frame sizes and layer bounds are both
expressed as integers, but frame sizes are in physical pixels and layer bounds
are in device-independent pixels. If the device scale is not an integer, there
may not be an exact mapping between the two spaces. For instance, if the layer
size is 105 x 211 and the device scale is 1.5, the delegated content is
responsible for filling 157.5 x 316.5 physical pixels in the final output. This
isn't really possible so what actually happens is the delegated renderer
produces a frame with a physical size of 158 x 317 pixels and sends that up to
the DRLI. To map this frame into physical space, the DRLI should apply the
inverse of the device scale factor to reflect that the frame covers 105.333 x
211.3333 logical pixels.

Before this patch, DRLI would attempt to scale the frame by (105/158, 211/317)
resulting in the frame being scaled down non-uniformly to try to make 158 x 317
pixels fit into a 157.5 x 316.6 space. This scaling looks bad and ends up
mapping pixels partially outside of the layer's logical bounds into the layer's
logical bounds.

BUG=370074

Review URL: https://codereview.chromium.org/311253004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275544 0039d316-1c4b-4281-b951-d872f2087c98