Add extension pinning to extensions menu

Adds a syncable extension preference for pinned extensions to keep them
in sync. This is used in place of the previous "visible extensions"
paradigm.

Notably extensions are not pinned by default, whereas before they were
initially visible on the toolbar.

Bug: chromium:959932
Change-Id: I23fa0d5314317c079841540e18449646f4da9076
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1598818
Commit-Queue: Peter Boström <[email protected]>
Reviewed-by: Devlin <[email protected]>
Cr-Commit-Position: refs/heads/master@{#665682}
diff --git a/extensions/browser/pref_names.h b/extensions/browser/pref_names.h
index f662e61..9aeeed5 100644
--- a/extensions/browser/pref_names.h
+++ b/extensions/browser/pref_names.h
@@ -88,6 +88,10 @@
 // Time of the next scheduled extensions auto-update checks.
 extern const char kNextUpdateCheck[];
 
+// A preference that tracks extensions pinned to the toolbar. This is a list
+// object stored in the Preferences file. The extensions are stored by ID.
+extern const char kPinnedExtensions[];
+
 // Indicates on-disk data might have skeletal data that needs to be cleaned
 // on the next start of the browser.
 extern const char kStorageGarbageCollect[];