Add a Extensions Menu skeleton implementation

Adds a flag which, when enabled, adds an ExtensionsToolbarButton to
ToolbarView. When clicked this triggers a bubble that currently just
lists the extensions (without images). Buttons in it cannot be
interacted with yet.

Bug: chromium:943702

Change-Id: I607c3dcb0efa0c1b413c7623b301dd6869bc3da9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1519096
Commit-Queue: Peter Boström <[email protected]>
Reviewed-by: Scott Violet <[email protected]>
Reviewed-by: Devlin <[email protected]>
Cr-Commit-Position: refs/heads/master@{#642791}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index 2828253a..ef7fe77 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -621,6 +621,9 @@
 extern const char kExtensionContentVerificationEnforce[];
 extern const char kExtensionContentVerificationEnforceStrict[];
 
+extern const char kExtensionsToolbarMenuName[];
+extern const char kExtensionsToolbarMenuDescription[];
+
 extern const char kExtensionsOnChromeUrlsName[];
 extern const char kExtensionsOnChromeUrlsDescription[];