gfx::Image: Made Image objects shallow-copy their representations.

gfx::Image: Made Image objects shallow-copy their representations.

Previously, Images kept a reference-counted map. Now they just clone the
entire map. All of the objects inside are reference-counted, so this is
slightly more expensive but not by much, and avoids some thread safety
issues.

PROOF OF CONCEPT / DO NOT SUBMIT

Problems with this approach:
- Breaks ToImageSkia(); the returned pointer now points into storage
  owned by the immediate Image object it was called on (previously it
  pointed at memory shared by all of the Image objects). Lots of client
  code relies on this and it is non-trivial to clean it up.
- Efficiency issues: This weakens the effects of caching because each
  individual image object has its own copy of the cache (e.g., imagine
  an Image with an ImageRepPNG is copied, then AsImageSkia() is called,
  then the copy is deleted; the original Image will not have an
  ImageSkia rep).

BUG=600237

[Matt Giuca, 2016-04-08 03:58:37]

Description was changed from

==========
gfx::Image: Made Image objects shallow-copy their representations.

Previously, Images kept a reference-counted map. Now they just clone the
entire map. All of the objects inside are reference-counted, so this is
slightly more expensive but not by much, and avoids some thread safety
issues.

PROOF OF CONCEPT / DO NOT SUBMIT

Problems with this approach:
- NSImage and UIImage are not refcounted (maybe they are internally, but
  here they are just raw pointers).
- Breaks ToImageSkia(); the returned pointer now points into storage
  owned by the immediate Image object it was called on (previously it
  pointed at memory shared by all of the Image objects). Lots of client
  code relies on this and it is non-trivial to clean it up.
- Efficiency issues: This weakens the effects of caching because each
  individual image object has its own copy of the cache (e.g., imagine
  an Image with an ImageRepPNG is copied, then AsImageSkia() is called,
  then the copy is deleted; the original Image will not have an
  ImageSkia rep).

BUG=600237
==========

to

==========
gfx::Image: Made Image objects shallow-copy their representations.

Previously, Images kept a reference-counted map. Now they just clone the
entire map. All of the objects inside are reference-counted, so this is
slightly more expensive but not by much, and avoids some thread safety
issues.

PROOF OF CONCEPT / DO NOT SUBMIT

Problems with this approach:
- Breaks ToImageSkia(); the returned pointer now points into storage
  owned by the immediate Image object it was called on (previously it
  pointed at memory shared by all of the Image objects). Lots of client
  code relies on this and it is non-trivial to clean it up.
- Efficiency issues: This weakens the effects of caching because each
  individual image object has its own copy of the cache (e.g., imagine
  an Image with an ImageRepPNG is copied, then AsImageSkia() is called,
  then the copy is deleted; the original Image will not have an
  ImageSkia rep).

BUG=600237
==========