Use for loop instead of while in GetExistingProcessHost

In RenderProcessHostImpl::GetExistingProcessHost, there is a loop
that enumerates all current process hosts. It is implement as a
while loop having iter.Advance() at the end.

Lets assume a situation when one decides to prohibit some process
host from being put to suitable_renderers list. The obvious solution
is to check some condition and use continue statement to move to
next loop iteration... which results in an infinite loop if
iter.Advance() isn't called prior to continue.

So it looks more safe to use for loop instead as it executes its
3rd part automatically reducing the human factor influence.

This CL is a pure refactoring so there is no behavior change at all.

Change-Id: I456cd2ff5348172419e8561d4ebf75d886a02e15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2071016
Commit-Queue: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Auto-Submit: Denis Bessonov <dbessonov@yandex-team.ru>
Cr-Commit-Position: refs/heads/master@{#744396}

content/browser/renderer_host/render_process_host_impl.cc

@@ -4093,8 +4093,7 @@ RenderProcessHost* RenderProcessHostImpl::GetExistingProcessHost(
   std::vector<RenderProcessHost*> suitable_renderers;
   suitable_renderers.reserve(GetAllHosts().size());
 
-  iterator iter(AllHostsIterator());
-  while (!iter.IsAtEnd()) {
+  for (iterator iter(AllHostsIterator()); !iter.IsAtEnd(); iter.Advance()) {
     if (iter.GetCurrentValue()->MayReuseHost() &&
         RenderProcessHostImpl::IsSuitableHost(
             iter.GetCurrentValue(), site_instance->GetIsolationContext(),
@@ -4107,7 +4106,6 @@ RenderProcessHost* RenderProcessHostImpl::GetExistingProcessHost(
 
       suitable_renderers.push_back(iter.GetCurrentValue());
     }
-    iter.Advance();
   }
 
   // Now pick a random suitable renderer, if we have any.