[bfcache] Rename DisableForRenderFrameHostForTesting

This CL renames DisableForRenderFrameHostForTesting to
DisableBFCacheForRFHForTesting to make clear what it does for
testing.

This is a follow-up CL of https://crrev.com/c/3056266.

Bug: 1233870
Change-Id: I3a6bb2a20b4387f785152bcc57e2760a9661c435
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3066703
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Julie Kim <jkim@igalia.com>
Cr-Commit-Position: refs/heads/master@{#908363}