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: refs/heads/master@{#338909}