Track down crash during RenderFrameImpl::didCommitProvisionalLoad.

It's possible that the RenderFrame isn't being deleted during swap
in some case, in which case the RenderView doesn't have its
main_render_frame_ cleared.

BUG=575245
TEST=Crashes in OnSwapOut instead of didCommitProvisionalLoad.

Review URL: https://codereview.chromium.org/1581193002

Cr-Commit-Position: refs/heads/master@{#369577}
1 file changed