Simplify URL-resolving in targetElementFromIRIString

Use Document::completeURL for URL resolving, rather than using
String::substring et.c to reproduce essentially the same code.
Use KURL::fragmentIdentifier and friends to extract the fragment
identifier.

Fold urlFromIRIStringWithFragmentIdentifier into its only user.
Open-code isExternalURIReference in targetElementFromIRIString, since we
already have the URL resolved and ready.

BUG=470608

Review-Url: https://codereview.chromium.org/2170453002
Cr-Commit-Position: refs/heads/master@{#406578}
1 file changed