Add "panel_fitting" GPU feature type and use it for mirror mode.

content/public/common/gpu_feature_type.h

Index: content/public/common/gpu_feature_type.h
diff --git a/content/public/common/gpu_feature_type.h b/content/public/common/gpu_feature_type.h
index 5900ec0a20e868b09d290175c50eb77131b7dbb6..5a698863969813ffd4bd6cfc31af65e3d8ed0bb4 100644
--- a/content/public/common/gpu_feature_type.h
+++ b/content/public/common/gpu_feature_type.h
@@ -19,6 +19,7 @@ enum GpuFeatureType {
   GPU_FEATURE_TYPE_FLASH_STAGE3D = 1 << 5,
   GPU_FEATURE_TYPE_TEXTURE_SHARING = 1 << 6,
   GPU_FEATURE_TYPE_ACCELERATED_VIDEO_DECODE = 1 << 7,
+  GPU_FEATURE_TYPE_PANEL_FITTING = 1 << 8,
   GPU_FEATURE_TYPE_ALL = GPU_FEATURE_TYPE_ACCELERATED_2D_CANVAS |
                          GPU_FEATURE_TYPE_ACCELERATED_COMPOSITING |
                          GPU_FEATURE_TYPE_WEBGL |
@@ -26,7 +27,8 @@ enum GpuFeatureType {
                          GPU_FEATURE_TYPE_FLASH3D |
                          GPU_FEATURE_TYPE_FLASH_STAGE3D |
                          GPU_FEATURE_TYPE_TEXTURE_SHARING |
-                         GPU_FEATURE_TYPE_ACCELERATED_VIDEO_DECODE,
+                         GPU_FEATURE_TYPE_ACCELERATED_VIDEO_DECODE |
+                         GPU_FEATURE_TYPE_PANEL_FITTING,
   // All flags initialized to false, i.e., no feature is blacklisted.
   GPU_FEATURE_TYPE_UNKNOWN = 0
 };