Revert of Revert of Enable ChannelMojo (patchset #1 id:1 of https://codereview.chromium.org/1002973002/)
Revert of Revert of Enable ChannelMojo (patchset #1 id:1 of https://codereview.chromium.org/1002973002/)

Reason for revert:
Suspecting that this is breaking a bunch of Phishing*, DomSerializer*, and ResourceFetcher* tests on various bots.  For example:
https://build.chromium.org/p/chromium.linux/builders/Linux%20Tests%20%28dbg%29%281%29%2832%29/builds/10977
and https://chromium.googlesource.com/chromium/src/+log/7798a5020aca61cf96eb783fbf0de201bd2b8553..e2a21a01b60eb329e61499458f51ebae7a0951ad?pretty=fuller

(blamelist intersection is r320394:r320395)

Sample output (*SerializeDocumentWithDownloadedIFrame):

[28371:28482:0312/162210:38108696320:FATAL:scoped_ipc_support.cc(33)] Check failed: (init_count_ == 0 && !io_thread_task_runner_) || io_thread_task_runner_ == io_thread_task_runner.
#0 0x0000f47ce8c4 base::debug::StackTrace::StackTrace()
#1 0x0000f482ff71 logging::LogMessage::~LogMessage()
#2 0x0000f2b503d3 IPC::(anonymous namespace)::IPCSupportInitializer::Init()
#3 0x0000f2b501cd IPC::ScopedIPCSupport::ScopedIPCSupport()
#4 0x0000e9e5ddf6 content::ChannelInit::Init()
#5 0x0000e9990577 content::MojoApplication::OnActivate()
#6 0x0000e9990cb1 _Z20DispatchToMethodImplIN7content15MojoApplicationEMS1_FvRKN4base14FileDescriptorEEJS3_EJLj0EEEvPT_T0_RK5TupleIJDpT1_EE13IndexSequenceIJXspT2_EEE
#7 0x0000e9990bd9 _Z16DispatchToMethodIN7content15MojoApplicationEMS1_FvRKN4base14FileDescriptorEEJS3_EEvPT_T0_RK5TupleIJDpT1_EE
#8 0x0000e99909ce MojoMsg_Activate::Dispatch\u003C>()
#9 0x0000e9990302 content::MojoApplication::OnMessageReceived()
#10 0x0000e99079ef content::ChildThreadImpl::OnMessageReceived()
#11 0x0000f31f7ee4 IPC::ChannelProxy::Context::OnDispatchMessage()

Original issue's description:
> Revert of Enable ChannelMojo (patchset #3 id:40001 of https://codereview.chromium.org/857483004/)
>
> Reason for revert:
> This is breaking a lot of things (He's dead jim pages, extensions, and other things). See issue 462026.
>
> Original issue's description:
> > Enable ChannelMojo
> >
> > As we addressed certain amount of the performance problem,
> > it's time to give it another try.
> >
> > R=viettrungluu@chromium.org
> > BUG=377980
> >
> > Committed: https://crrev.com/a06cd1b2fe83b51b0874ca504066f00a17192026
> > Cr-Commit-Position: refs/heads/master@{#313184}
>
> TBR=viettrungluu@chromium.org,morrita@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=377980
>
> Committed: https://crrev.com/779310e250bee978cc6aaf68ef4dede4bd742357
> Cr-Commit-Position: refs/heads/master@{#320394}

TBR=viettrungluu@chromium.org,morrita@chromium.org,skuhne@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=377980

Review URL: https://codereview.chromium.org/1000173003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#320410}

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