Revert of Do not create proxy hosts in the subtree of navigating frame. (patchset...

Revert of Do not create proxy hosts in the subtree of navigating frame. (patchset #11 id:200001 of https://codereview.chromium.org/536143002/)

Reason for revert:
SitePerProcessBrowserTest.ProxyCreationSkipsSubtree repeatedly fails on Mac with ASAN reporting use-after-free, see:

http://build.chromium.org/p/chromium.memory/builders/Mac%20ASan%2064%20Tests%20%281%29/builds/2174
http://build.chromium.org/p/chromium.fyi/builders/Chromium%20Mac%2010.8%20Buildrunner/builds/19009

@Nasko, as there are some simultaneous breakages right now, so I could not investigate this further and opted to revert the whole thing to get the tree green ASAP.

Snippets from test log:

ASSERTION FAILED: !localFrame || (localFrame->isLocalFrame())
../../third_party/WebKit/Source/core/frame/LocalFrame.h(253) : blink::LocalFrame *blink::toLocalFrame(blink::Frame *)
../../content/browser/site_per_process_browsertest.cc:690: Failure
Value of: child->child_count()
  Actual: 2
Expected: 0U
Which is: 0

Original issue's description:
> Do not create proxy hosts in the subtree of navigating frame.
>
> When a frame is navigating cross-process, its existing document will be completely destroyed and all child frames will be gone. This means that we don't need to create proxy objects for the new SiteInstance in the subtree of the navigating frame.
>
> BUG=357747
>
> Committed: https://crrev.com/faab4288197b20b73610aad3e5579751b0c3a97b
> Cr-Commit-Position: refs/heads/master@{#294520}

TBR=ajwong@chromium.org,kenrb@chromium.org,jam@chromium.org,creis@chromium.org,dmazzoni@chromium.org,nasko@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=357747

Review URL: https://codereview.chromium.org/565103002

Cr-Commit-Position: refs/heads/master@{#294552}