Add onMemoryDump to GrContext

tools/BUILD.public.expected

 This file is auto-generated by tools/BUILD_simulator.py.
 It expands BUILD.public to make it easy to see changes.
 COPTS = ['-Wno-deprecated-declarations', '-Wno-implicit-fallthrough']
 DEFINES = ['SK_RELEASE',
  'SK_USE_FLOATBITS',
  'SK_USE_FREETYPE_EMBOLDEN',
  'GOOGLE3',
  'SK_BUILD_FOR_UNIX',
  'SK_SAMPLES_FOR_X']
 HDRS = ['include/c/sk_canvas.h',
@@ skipping 656 matching lines @@
  'src/gpu/gl/GrGLPathProgramDataManager.cpp',
  'src/gpu/gl/GrGLPathRange.cpp',
  'src/gpu/gl/GrGLPathRendering.cpp',
  'src/gpu/gl/GrGLPrimitiveProcessor.cpp',
  'src/gpu/gl/GrGLProgram.cpp',
  'src/gpu/gl/GrGLProgramDataManager.cpp',
  'src/gpu/gl/GrGLProgramDesc.cpp',
  'src/gpu/gl/GrGLRenderTarget.cpp',
  'src/gpu/gl/GrGLStencilAttachment.cpp',
  'src/gpu/gl/GrGLTexture.cpp',
+ 'src/gpu/gl/GrGLTextureRenderTarget.cpp'
  'src/gpu/gl/GrGLUtil.cpp',
  'src/gpu/gl/GrGLVertexArray.cpp',
  'src/gpu/gl/GrGLVertexBuffer.cpp',
  'src/gpu/gl/GrGLXferProcessor.cpp',
  'src/gpu/gl/SkGLContext.cpp',
  'src/gpu/gl/SkNullGLContext.cpp',
  'src/gpu/gl/builders/GrGLFragmentShaderBuilder.cpp',
  'src/gpu/gl/builders/GrGLGeometryShaderBuilder.cpp',
  'src/gpu/gl/builders/GrGLPathProgramBuilder.cpp',
  'src/gpu/gl/builders/GrGLProgramBuilder.cpp',
@@ skipping 146 matching lines @@
  'src/utils/SkTextureCompressor.cpp',
  'src/utils/SkTextureCompressor_ASTC.cpp',
  'src/utils/SkTextureCompressor_LATC.cpp',
  'src/utils/SkTextureCompressor_R11EAC.cpp',
  'src/utils/SkThreadUtils_pthread.cpp',
  'src/utils/SkThreadUtils_pthread_linux.cpp',
  'third_party/etc1/etc1.cpp',
  'third_party/ktx/ktx.cpp']
 SRCS_SSE4 = ['src/opts/SkBlitRow_opts_SSE4.cpp', 'src/opts/SkOpts_sse41.cpp']
 SRCS_SSSE3 = ['src/opts/SkBitmapProcState_opts_SSSE3.cpp', 'src/opts/SkOpts_ssse3.cpp']