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Skia cannot use Chromium's implementation of mutex (Lock) due to static
initializers. However, we would like to be able to use Chromium's
implementation of atomics. This motivates the split of implementation.

Skia's atomic and mutex calls should be inlinable, especially the atomics.
These calls often compile down to very few instructions, and we currently have
the overhead of a function call. This motivates the header implementation.

There is still a desire for the build system to select the implementation, so
the SK_XXX_PLATFORM_H pattern for header files is introduced. This allows the
build system to control which platform specific header files are chosen.

The Chromium side changes (most of which will need to go in before this change
can be found at https://codereview.chromium.org/19477005/ .
The Chromium side changes after this lands can be seen at 
https://codereview.chromium.org/98073013 .

Review URL: https://codereview.chromium.org/19808007

git-svn-id: http://skia.googlecode.com/svn/trunk@12738 2bbb7eff-a529-9590-31e7-b0007b416f81