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 87517df09f55e938a391b6aac308581828ffcafd..a862af3ff5fdfe514bd60743ea15b27b1d875fbe 100644
--- a/content/public/common/gpu_feature_type.h
+++ b/content/public/common/gpu_feature_type.h
@@ -21,6 +21,7 @@ enum GpuFeatureType {
   GPU_FEATURE_TYPE_ACCELERATED_VIDEO_DECODE = 1 << 7,
   GPU_FEATURE_TYPE_3D_CSS = 1 << 8,
   GPU_FEATURE_TYPE_ACCELERATED_VIDEO = 1 << 9,
+  GPU_FEATURE_TYPE_PANEL_FITTING = 1 << 10,
   GPU_FEATURE_TYPE_ALL = GPU_FEATURE_TYPE_ACCELERATED_2D_CANVAS |
                          GPU_FEATURE_TYPE_ACCELERATED_COMPOSITING |
                          GPU_FEATURE_TYPE_WEBGL |
@@ -30,7 +31,8 @@ enum GpuFeatureType {
                          GPU_FEATURE_TYPE_TEXTURE_SHARING |
                          GPU_FEATURE_TYPE_ACCELERATED_VIDEO_DECODE |
                          GPU_FEATURE_TYPE_3D_CSS |
-                         GPU_FEATURE_TYPE_ACCELERATED_VIDEO,
+                         GPU_FEATURE_TYPE_ACCELERATED_VIDEO |
+                         GPU_FEATURE_TYPE_PANEL_FITTING,
   // All flags initialized to false, i.e., no feature is blacklisted.
   GPU_FEATURE_TYPE_UNKNOWN = 0
 };