Add support for using alternative backends (like DirectX) when creating a GrGpu.

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 82 matching lines @@
       '<(skia_src_path)/gpu/GrGeometryBuffer.h',
       '<(skia_src_path)/gpu/GrGeometryProcessor.h',
       '<(skia_src_path)/gpu/GrGeometryProcessor.cpp',
       '<(skia_src_path)/gpu/GrGlyph.h',
       '<(skia_src_path)/gpu/GrGpu.cpp',
       '<(skia_src_path)/gpu/GrGpu.h',
       '<(skia_src_path)/gpu/GrGpuResourceCacheAccess.h',
       '<(skia_src_path)/gpu/GrGpuResourcePriv.h',
       '<(skia_src_path)/gpu/GrGpuResource.cpp',
       '<(skia_src_path)/gpu/GrGpuFactory.cpp',
+      '<(skia_src_path)/gpu/GrGpuFactory.h',
       '<(skia_src_path)/gpu/GrIndexBuffer.h',
       '<(skia_src_path)/gpu/GrInvariantOutput.cpp',
       '<(skia_src_path)/gpu/GrInOrderDrawBuffer.cpp',
       '<(skia_src_path)/gpu/GrInOrderDrawBuffer.h',
       '<(skia_src_path)/gpu/GrLayerCache.cpp',
       '<(skia_src_path)/gpu/GrLayerCache.h',
       '<(skia_src_path)/gpu/GrLayerHoister.cpp',
       '<(skia_src_path)/gpu/GrLayerHoister.h',
       '<(skia_src_path)/gpu/GrMemoryPool.cpp',
       '<(skia_src_path)/gpu/GrMemoryPool.h',
@@ skipping 275 matching lines @@
       '<(skia_src_path)/gpu/gl/debug/SkDebugGLContext.cpp',
       '<(skia_src_path)/gpu/gl/debug/SkDebugGLContext.h',
     ],
     'skgpu_null_gl_sources': [
       '<(skia_src_path)/gpu/gl/GrGLCreateNullInterface.cpp',
       '<(skia_src_path)/gpu/gl/SkNullGLContext.cpp',
       '<(skia_include_path)/gpu/gl/SkNullGLContext.h',
     ],
   },
 }