Extensions: Correct documentation for GetPermittedChromeSchemeHosts.

ExtensionsClient::GetPermittedChromeSchemeHosts is only used for determining
valid host permissions and is not used for determining valid scriptable hosts.
Correct the function documentation to reflect the same.

BUG=None

Change-Id: I45b171132be5871193d73ad535e9bd6111abbd4b
Reviewed-on: https://chromium-review.googlesource.com/886840Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532169}

extensions/common/extensions_client.h

@@ -80,7 +80,8 @@ class ExtensionsClient {
   // any origin.
   virtual const ScriptingWhitelist& GetScriptingWhitelist() const = 0;
 
-  // Get the set of chrome:// hosts that |extension| can run content scripts on.
+  // Get the set of chrome:// hosts that |extension| can have host permissions
+  // for.
   virtual URLPatternSet GetPermittedChromeSchemeHosts(
       const Extension* extension,
       const APIPermissionSet& api_permissions) const = 0;