Make EnableViewSourceMode a FrameMsg

This CL moves the EnableViewSourceMode from ViewMsgs to FrameMsgs. This is a
preparatory CL for https://codereview.chromium.org/1811913003/.

BUG=555418
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_site_isolation

Review URL: https://codereview.chromium.org/1832803002

Cr-Commit-Position: refs/heads/master@{#383062}
diff --git a/content/renderer/render_frame_impl.h b/content/renderer/render_frame_impl.h
index 8aa008d..6c09c8d1 100644
--- a/content/renderer/render_frame_impl.h
+++ b/content/renderer/render_frame_impl.h
@@ -793,6 +793,7 @@
               const base::string16& search_text,
               const blink::WebFindOptions& options);
   void OnStopFinding(StopFindAction action);
+  void OnEnableViewSourceMode();
 #if defined(OS_ANDROID)
   void OnActivateNearestFindResult(int request_id, float x, float y);
   void OnFindMatchRects(int current_version);