Add usage guideline comments for custom GL context priority.

ui/gl/gl_context.h

Index: ui/gl/gl_context.h
diff --git a/ui/gl/gl_context.h b/ui/gl/gl_context.h
index e72b1ecf817531e0e6a536042cad622fe35e933d..69f4c384825fd9af81282093912030069440fd9d 100644
--- a/ui/gl/gl_context.h
+++ b/ui/gl/gl_context.h
@@ -39,6 +39,19 @@ struct GLVersionInfo;
 class RealGLApi;
 class TraceGLApi;
 
+// Where available, choose a GL context priority for devices that support it.
+// Currently this requires the EGL_IMG_context_priority extension that is
+// present on Daydream ready Android devices. Default is Medium, and the
+// attribute is ignored if the extension is missing.
+//
+// "High" priority must only be used for special cases with strong realtime
+// requirements, it is incompatible with other critical system GL work such as
+// the GVR library's asynchronous reprojection for VR viewing. Please avoid
+// using it for any GL contexts that may be used during VR presentation,
+// see crbug.com/727800.
+//
+// Instead, consider using "Low" priority for possibly-slow GL work such as
+// user WebGL content.
 enum ContextPriority {
   ContextPriorityLow,
   ContextPriorityMedium,