[Mac] Add system share menu

NB: This is continued from https://codereview.chromium.org/2950403002/
Landing it behind a flag now to catch M63 feature freeze.

Replaces the "Email Link to Page" item in the file menu with a submenu
that allows sharing via macOS Sharing Extensions. The previous e-mail
command is superseded by the Mail sharing service, which inherits the
key command.

Bug: 465302
Change-Id: Ie3422389118d6146df5cc2fbe8610046e3b6c563
Reviewed-on: https://chromium-review.googlesource.com/693178
Reviewed-by: Robert Sesek <[email protected]>
Commit-Queue: Leonard Grey <[email protected]>
Commit-Queue: Aaron Gable <[email protected]>
Cr-Commit-Position: refs/heads/master@{#506117}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 39da50c..5d26322 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -1380,6 +1380,9 @@
 extern const char kMacRTLName[];
 extern const char kMacRTLDescription[];
 
+extern const char kMacSystemShareMenuName[];
+extern const char kMacSystemShareMenuDescription[];
+
 extern const char kMacTouchBarName[];
 extern const char kMacTouchBarDescription[];