skia: add ability to load command_buffer_gles2

BUG=skia:

Review URL: https://codereview.chromium.org/1306823003
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)',