Avoid rewriting about:srcdoc into chrome://srcdoc

Rewriting about:srcdoc into chrome://srcdoc is undesirable because

1. about:srcdoc has a special meaning and just like about:blank has been
   reserved by specs like
   https://html.spec.whatwg.org/multipage/urls-and-fetching.html

2. chrome:-scheme URLs are special and might have extra privileges.
   Therefore chrome: URLs should not be reachable by an unprivileged webpage
   (OTOH, the rewriting fixed here only applies to the URL *shown* to
   the user, not the URL that gets committed - compare WebContents's
   GetVisibleURL vs GetLastCommittedURL).

Bug: 973628
Change-Id: I021e623caf0d7e5c02a2546291bb4913412b3125
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1654909
Auto-Submit: Łukasz Anforowicz <[email protected]>
Commit-Queue: Łukasz Anforowicz <[email protected]>
Commit-Queue: Avi Drissman <[email protected]>
Reviewed-by: Avi Drissman <[email protected]>
Reviewed-by: Charlie Harrison <[email protected]>
Reviewed-by: Peter Kasting <[email protected]>
Cr-Commit-Position: refs/heads/master@{#669328}
8 files changed