Move SkChecksum::Murmur3 out of the header.

As we use this more and more, we end up with more and more inline copies.
On my desktop, this makes Skia ~16K smaller.

Boy perf trybots would be neat.

BUG=skia:

Review URL: https://codereview.chromium.org/1415133003
diff --git a/gyp/core.gypi b/gyp/core.gypi
index c71ea31..85bc351 100644
--- a/gyp/core.gypi
+++ b/gyp/core.gypi
@@ -65,6 +65,7 @@
         '<(skia_src_path)/core/SkCachedData.cpp',
         '<(skia_src_path)/core/SkCanvas.cpp',
         '<(skia_src_path)/core/SkCanvasPriv.h',
+        '<(skia_src_path)/core/SkChecksum.cpp',
         '<(skia_src_path)/core/SkChunkAlloc.cpp',
         '<(skia_src_path)/core/SkClipStack.cpp',
         '<(skia_src_path)/core/SkColor.cpp',