blink: converts KURL::GetString() to return AtomicString

This avoids unnecessary operations in some places. For example,
without this, operator==(const KURL&, const KURL&) was using
operator==(const String&, const String&), which is not as efficient
as operator==(const AtomicString&, const AtomicString&).

Bug: None
Change-Id: I2797f86210ec7d193aa9100fa77115ec4b6f9635
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5191546
Commit-Queue: Scott Violet <sky@chromium.org>
Auto-Submit: Scott Violet <sky@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1252352}