CustomHandler: Remove testing folder in navigatorcontentutils

The testing folder was to test *isRegisterProtocolHandler* mainly. But, the function
was removed from the spec. And also, the implementation was removed as well. So, this CL
removes testing folder for now. But, we can restore the folder if isProtocolHandlerRegistered
can be implemented in blink.

Bug: 121825
Change-Id: Ia3ff296ddf93adbf0ac39362c31d83de9d995113
Reviewed-on: https://chromium-review.googlesource.com/c/1391040
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Cr-Commit-Position: refs/heads/master@{#619059}