skia: add ability to load command_buffer_gles2

BUG=skia:

Review URL: https://codereview.chromium.org/1306823003
diff --git a/gyp/common.gypi b/gyp/common.gypi
index f822166..a3ce66e 100644
--- a/gyp/common.gypi
+++ b/gyp/common.gypi
@@ -72,6 +72,16 @@
           ],
         },
       }],
+      [ 'skia_command_buffer', {
+        'defines': [
+          'SK_COMMAND_BUFFER',
+        ],
+        'direct_dependent_settings': {
+          'defines': [
+            'SK_COMMAND_BUFFER',
+          ],
+        },
+      }],
       [ 'skia_win_debuggers_path and skia_os == "win"',
         {
           'defines': [
diff --git a/gyp/common_variables.gypi b/gyp/common_variables.gypi
index 715f08e..23ce00a 100644
--- a/gyp/common_variables.gypi
+++ b/gyp/common_variables.gypi
@@ -129,6 +129,7 @@
       'skia_resource_cache_mb_limit%': 0,
       'skia_resource_cache_count_limit%': 0,
       'skia_angle%': 0,
+      'skia_command_buffer%': 0,
       'skia_gdi%': 0,
       'skia_gpu%': 1,
       'skia_osx_deployment_target%': '',
@@ -206,6 +207,7 @@
     'skia_angle%': '<(skia_angle)',
     'skia_arch_type%': '<(skia_arch_type)',
     'skia_chrome_utils%': '<(skia_chrome_utils)',
+    'skia_command_buffer%': '<(skia_command_buffer)',
     'skia_gdi%': '<(skia_gdi)',
     'skia_gpu%': '<(skia_gpu)',
     'skia_vulkan%': '<(skia_vulkan)',
diff --git a/gyp/gpu.gyp b/gyp/gpu.gyp
index 51e7e32..4179e1f 100644
--- a/gyp/gpu.gyp
+++ b/gyp/gpu.gyp
@@ -103,6 +103,7 @@
         '<@(skgpu_sources)',
         '<@(skgpu_native_gl_sources)',
         '<@(skgpu_angle_gl_sources)',
+        '<@(skgpu_command_buffer_gl_sources)',
         '<@(skgpu_mesa_gl_sources)',
         '<@(skgpu_debug_gl_sources)',
         '<@(skgpu_null_gl_sources)',
@@ -211,6 +212,12 @@
             '<@(skgpu_angle_gl_sources)',
           ],
         }],
+        [ 'skia_command_buffer', {
+        }, { # not skia_command_buffer
+          'sources!': [
+            '<@(skgpu_command_buffer_gl_sources)',
+          ],
+        }],
         [ 'skia_os == "android"', {
           'sources!': [
             '../src/gpu/gl/GrGLDefaultInterface_none.cpp',
diff --git a/gyp/gpu.gypi b/gyp/gpu.gypi
index 7750e7b..60ac5eb 100644
--- a/gyp/gpu.gypi
+++ b/gyp/gpu.gypi
@@ -432,6 +432,11 @@
       '<(skia_include_path)/gpu/gl/angle/SkANGLEGLContext.h',
       '<(skia_src_path)/gpu/gl/angle/SkANGLEGLContext.cpp',
     ],
+    'skgpu_command_buffer_gl_sources': [
+      # Sk files
+      '<(skia_include_path)/gpu/gl/command_buffer/SkCommandBufferGLContext.h',
+      '<(skia_src_path)/gpu/gl/command_buffer/SkCommandBufferGLContext.cpp',
+    ],
     'skgpu_debug_gl_sources': [
       '<(skia_src_path)/gpu/gl/debug/GrGLCreateDebugInterface.cpp',
       '<(skia_src_path)/gpu/gl/debug/GrFakeRefObj.h',