Add a CustomSchemeHandler runtime flag to remove ENABLE(CUSTOM_SCHEME_HANDLER).

As a step to remove #ifdef, this cl add a new runtime flag(CustomSchemeHandler), then it removes #if ENABLE(CUSTOM_SCHEME_HANDLER).

BUG=N/A

Review URL: https://codereview.chromium.org/22859043

git-svn-id: svn://svn.chromium.org/blink/trunk@164740 bbb929c8-8fbe-4397-9dbb-9b2b20218538