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This CL is desirable today, to avoid the memory leaks associated with
queueing of IPCs that are sent to RenderProcessHosts that are
constructed, but not yet initialized - such RenderProcessHosts
accumulate IPC messages and flush them only after renderer process is
actually launched at a later point / after a call to RPH::Init.
This aspect of the CL is very similar to r562797 which landed earlier.

This CL is desirable for the future - a tentative long-term plan for
https://crbug.com/813045 is to avoid creating an IPC channel before
RPH::Init is called.  This means that in the long-term
RPH::GetChildIdentity will crash (dereferencing a nullptr
|RenderProcessHostImpl::child_connection_|) if called on an
uninitialized RPH.  This CL ensures that this crash won't happen in
ClientSideDetectionService::SendModelToProcess and
SpellcheckService::OnCustomDictionaryChanged by ensuring that these
methods only work with already initialized RPHs.

The changes under //chrome/browser/spellchecker and
//chrome/browser/safe_browsing skip uninitialized RPHs for 2 specific
IPCs/subsystems.  The changes under //content/browser/renderer_host
prevent future IPCs/subsystems from using uninitialized RPHs, by
adding a DCHECK to RenderProcessHostImpl::GetChildIdentity.
The new DCHECK necessitates setting |is_initialized_ = true| much
earlier in RenderProcessHostImpl::Init (because
ChromeContentBrowserClient::RenderProcessWillLaunch needs to call
RPH::GetChildIdentity).

Bug: 813045
Change-Id: I4783de85cb7c199ab360f8361acd54c63287e1df
Reviewed-on: https://chromium-review.googlesource.com/1095433Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Jialiu Lin <jialiul@chromium.org>
Reviewed-by: Rachel Blum <groby@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567099}