Rename Frame::PrepareForCommit

Following
https://chromium-review.googlesource.com/c/chromium/src/+/1609783, this
function has a slightly modified behavior. Renamed it DetachDocument to
make it clearer what it is now doing.

Bug: 855189
Change-Id: I1c40f1019160529fdeaa10f04c71cdff456b1d32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1685381
Commit-Queue: Camille Lamy <clamy@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676882}
10 files changed