Forward declare blink::HitTestRequest

Reduce the number of translation units that include hit_test_request.h
from ~2800 to ~20. Change the public HitTestInDocument() to remove the
HitTestRequest parameter, as all external callers use the default
arguments anyway. Rename the existing HitTestInDocument() to
HitTestInDocumentImpl().

Change-Id: I8dffd00058639bd2ca4203365561c3b86b2c7238
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7149165
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1544290}