Mark FrameHost::deviceScaleFactor as deprecated.

This will be gone when transition is completed, but still needed for traditional mode. I added the comment when (not) to use it, and renamed to deviceScaleFactorDeprecated().

I'll create a similar CL for Page::deviceScaleFactor() later.

BUG=485650

Review-Url: https://codereview.chromium.org/1996223003
Cr-Commit-Position: refs/heads/master@{#395219}
7 files changed