fixup! [Custom Scheme] Restore isProtocolHandlerRegistered in historical.window.js

Issue 784653 removed isProtocolHandlerRegistered from a test that was ensuring
it's not supported. So this CL restores it and removed expected result as well.

Bug: 121825

Change-Id: I50609018d2349b584cccc4db1054ff4fe7447e43
Reviewed-on: https://chromium-review.googlesource.com/789712
Reviewed-by: Robert Ma <[email protected]>
Commit-Queue: Gyuyoung Kim <[email protected]>
Cr-Commit-Position: refs/heads/master@{#519459}

Author: Gyuyoung Kim

html/webappapis/system-state-and-capabilities/the-navigator-object/historical.window.js

@@ -1,5 +1,6 @@
 [
   "registerContentHandler",
+  "isProtocolHandlerRegistered",
   "isContentHandlerRegistered",
   "unregisterContentHandler"
 ].forEach(method => {