mac: Fix SkiaUtilsMacTest.NSImageRepToSkBitmap.
mac: Fix SkiaUtilsMacTest.NSImageRepToSkBitmap.

This test used the function CreateNSImage(), which first created an NSImage, and
then attached a NSBitmapImageRep to the NSImage. The problem is that the
NSBitmapImageRep was implicitly created, and its colorspace depended on the
linked version of OSX and hardware on the machine. I changed the implementation
of CreateNSImage() to use an explicitly created NSBitmapImageRep. I also changed
the colorspace of the function CreateNSImage() and the unit test to always be
+[NSColorSpace genericRGBColorSpace], to avoid potential wacky scenarios that
could arise with +[NSColorSpace deviceRGBColorSpace].

BUG=463170

Review URL: https://codereview.chromium.org/1225233004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#338909}

Pluto-tv/chromium-crosswalk,Just-D/chromium-1,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,Chilledheart/chromium,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,Just-D/chromium-1,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,Just-D/chromium-1,axinging/chromium-crosswalk,Chilledheart/chromium,Just-D/chromium-1,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Chilledheart/chromium,Just-D/chromium-1,Just-D/chromium-1,Chilledheart/chromium,Just-D/chromium-1,axinging/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Chilledheart/chromium