Move Webstore URL concepts to //extensions and out
of Chrome-specific "constants."

These URLs can be overridden by extensions embedders now
and are no longer incorrectly classified as "constants."

BUG=398671
[email protected],[email protected] for various files with header updates

Review URL: https://codereview.chromium.org/575113002

Cr-Commit-Position: refs/heads/master@{#295523}
diff --git a/chrome/browser/extensions/extension_system_impl.cc b/chrome/browser/extensions/extension_system_impl.cc
index cb953c9..ca34198 100644
--- a/chrome/browser/extensions/extension_system_impl.cc
+++ b/chrome/browser/extensions/extension_system_impl.cc
@@ -61,6 +61,7 @@
 #include "extensions/browser/warning_set.h"
 #include "extensions/common/constants.h"
 #include "extensions/common/extension.h"
+#include "extensions/common/extension_urls.h"
 #include "extensions/common/manifest.h"
 #include "net/base/escape.h"