Move mock gpu and caps, add Gr prefix, and create via GrContext::Create

Change-Id: I02427839f78372e337ed717b071c6baaa8ba4450
Reviewed-on: https://skia-review.googlesource.com/20622
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/gn/gpu.gni b/gn/gpu.gni
index 67e516a..e4ffe2f 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -458,6 +458,9 @@
   "$_src/gpu/glsl/GrGLSLXferProcessor.cpp",
   "$_src/gpu/glsl/GrGLSLXferProcessor.h",
 
+  # Mock
+  "$_src/gpu/mock/GrMockGpu.h",
+
   # Sk files
   "$_src/gpu/SkGpuDevice.cpp",
   "$_src/gpu/SkGpuDevice.h",