Extract an independent webkit_support_gfx target.
The new webkit_support_gfx, used by DumpRenderTree and ImageDiff,
doesn't depend on most other parts of chromium. This could make building
the individual target of ImageDiff fast. It's implemented by duplicating
most code in ui/gfx/codec/png_codec.cc, after removing code related to
This change is required to make ImageDiff for Android, and can benefit
other platforms, too. On Android, most targets are built for
toolsets:target. while ImageDiff needs to be build for toolsets:host.
Currently, building the standalone target of ImageDiff depends on about
85 other targets (try 'make ImageDiff'), which is inefficient. After the
change, ImageDiff only depends on webkit_support_gfx, libpng, and