Grant independence to SkBaseMutex on Windows.

Grant independence to SkBaseMutex on Windows.

Under POSIX, class SkMutex inherits from struct SkBaseMutex.
Since we can't have a POD mutex on Windows, we used to just
typedef SkMutex SkBaseMutex there. However, that makes it
impossible to forward-declare SkBaseMutex consistently across
platforms. With this CL we declare an empty struct SkBaseMutex
on Windows, which should have 0 cost but make the compiler happy.

R=bungeman@google.com, mtklein@google.com, tomhudson@google.com, bungeman, mtklein
BUG=skia:

Author: tomhudson@chromium.org

Review URL: https://codereview.chromium.org/364473002