Update TODO about hiding the main frame RenderWidget when swapping out

Combine 2 TODOs into one explanation about why this is done for main
frames only (subframes don't need it since we drop the RenderWidget
when the frame is detached during SwapOut).

R=creis@chromium.org

Change-Id: Ib10c51401994f9a3a6d8270b45eb1006297ecb50
Bug: 908582
Reviewed-on: https://chromium-review.googlesource.com/c/1362104
Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614523}
1 file changed