Delete entry in WebFrame->RenderFrameProxy map at WebFrame::close time.

The entry in the WebFrame->RenderFrameProxy map is removed in the
RenderFrameProxy destructor, but the destructor runs after the frame
has been destroyed. Remove the entry at the time of WebFrame::close().

BUG=357747

Review URL: https://codereview.chromium.org/1136593003

Cr-Commit-Position: refs/heads/master@{#328897}
2 files changed