Fix null dereference in RphFromRequest

Fix a null dereference when webrtcLoggingPrivate.upload() is called with
target_webview true, but where there's no actual webview to target.

Bug: 1194885
Change-Id: I460df83c9ffa2b8bf4156029cc38d664cfa60be5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3021387
Commit-Queue: Tony Herre <toprice@chromium.org>
Auto-Submit: Tony Herre <toprice@chromium.org>
Reviewed-by: Elad Alon <eladalon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#900964}