[Custom Scheme] Remove isProtocolHandlerRegistered in the NavigatorContentUtils of blink

isProtocolHandlerRegistered has been existed in blink. However, latest custom scheme whatwg
specfication removed it.

- whatwg: https://html.spec.whatwg.org/multipage/system-state.html#custom-handlers
- whatwg issue: https://github.com/whatwg/html/issues/630
- commit to remove it: https://github.com/whatwg/html/commit/b143dbc2d16f3473fcadee377d838070718549d3

The reason to remove it was that no browsers have supported the function. So it would be good
if we remove it from blink in order to reduce our maintanance overhead.

Bug: 121825
Change-Id: I3e99dd5996e55dedcd8b5d47a7103410acfd24eb
Reviewed-on: https://chromium-review.googlesource.com/784653
Commit-Queue: Gyuyoung Kim <gyuyoung.kim@lge.com>
Reviewed-by: Mike Lawther <mikelawther@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518981}
20 files changed