Reland: Add support for entering/exiting HTML fullscreen from OOPIFs.

content/common/frame_messages.h

Index: content/common/frame_messages.h
diff --git a/content/common/frame_messages.h b/content/common/frame_messages.h
index 7dd4a6480e9b2dbc75462d5c3a16f626ad68944a..bfdf41df1617f292a3eb18d5cf1e44b57dfe2c92 100644
--- a/content/common/frame_messages.h
+++ b/content/common/frame_messages.h
@@ -807,6 +807,12 @@ IPC_MESSAGE_ROUTED2(FrameMsg_DidUpdateOrigin,
 // support cross-process focused frame changes.
 IPC_MESSAGE_ROUTED0(FrameMsg_SetFocusedFrame)
 
+// Sent to a frame proxy when its real frame is preparing to enter fullscreen
+// in another process.  Actually entering fullscreen will be done separately as
+// part of ViewMsg_Resize, once the browser process has resized the tab for
+// fullscreen.
+IPC_MESSAGE_ROUTED0(FrameMsg_WillEnterFullscreen)
+
 // Send to the RenderFrame to set text tracks state and style settings.
 // Sent for top-level frames.
 IPC_MESSAGE_ROUTED1(FrameMsg_SetTextTrackSettings,