Add specialized content key class for resources.

gyp/gpu.gypi

 # Include this gypi to include all 'gpu' files
 # The parent gyp/gypi file must define
 #       'skia_src_path'     e.g. skia/trunk/src
 #       'skia_include_path' e.g. skia/trunk/include
 #
 # The skia build defines these in common_variables.gypi
 #
 {
   'variables': {
     'skgpu_sources': [
@@ skipping 44 matching lines @@
       '<(skia_src_path)/gpu/GrAddPathRenderers_default.cpp',
       '<(skia_src_path)/gpu/GrAllocator.h',
       '<(skia_src_path)/gpu/GrAtlas.cpp',
       '<(skia_src_path)/gpu/GrAtlas.h',
       '<(skia_src_path)/gpu/GrBitmapTextContext.cpp',
       '<(skia_src_path)/gpu/GrBitmapTextContext.h',
       '<(skia_src_path)/gpu/GrBlend.cpp',
       '<(skia_src_path)/gpu/GrBlend.h',
       '<(skia_src_path)/gpu/GrBufferAllocPool.cpp',
       '<(skia_src_path)/gpu/GrBufferAllocPool.h',
-      '<(skia_src_path)/gpu/GrCacheID.cpp',
       '<(skia_src_path)/gpu/GrClipData.cpp',
       '<(skia_src_path)/gpu/GrClipMaskCache.h',
       '<(skia_src_path)/gpu/GrClipMaskCache.cpp',
       '<(skia_src_path)/gpu/GrClipMaskManager.h',
       '<(skia_src_path)/gpu/GrClipMaskManager.cpp',
       '<(skia_src_path)/gpu/GrContext.cpp',
       '<(skia_src_path)/gpu/GrCoordTransform.cpp',
       '<(skia_src_path)/gpu/GrDefaultGeoProcFactory.cpp',
       '<(skia_src_path)/gpu/GrDefaultGeoProcFactory.h',
       '<(skia_src_path)/gpu/GrDefaultPathRenderer.cpp',
@@ skipping 290 matching lines @@
     ],
     'skgpu_null_gl_sources': [
       '<(skia_src_path)/gpu/gl/GrGLCreateNullInterface.cpp',
 
       # Sk files
       '<(skia_src_path)/gpu/gl/SkNullGLContext.cpp',
       '<(skia_src_path)/gpu/gl/SkNullGLContext.h',
     ],
   },
 }