Forward declare DocumentLoader in deprecation.h

Shrinks page.h by ~1.1M.

Cq-Include-Trybots: luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: Ie71224313e0a1c26319854707b14e62e65dc46a3
Reviewed-on: https://chromium-review.googlesource.com/1203911
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#588504}
3 files changed