Create public cpp directory for Message Center, move constants and flags
there.

Bug: 755413
Change-Id: I8d804a160e33b8667942aa723e031beb3d67bf8e
Reviewed-on: https://chromium-review.googlesource.com/651568
Commit-Queue: Evan Stade <[email protected]>
Reviewed-by: Scott Violet <[email protected]>
Reviewed-by: James Cook <[email protected]>
Reviewed-by: Peter Beverloo <[email protected]>
Cr-Commit-Position: refs/heads/master@{#500688}
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index 8c5855e..dacd920 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -1764,6 +1764,7 @@
     "//ui/gl",
     "//ui/gl:gl_features",
     "//ui/message_center",
+    "//ui/message_center/public/cpp",
     "//ui/resources",
     "//ui/shell_dialogs",
     "//ui/strings",