Fix is pending: https://codereview.chromium.org/2961423002/

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The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6c760df2f6a9f5d1ce7051be6acfeebc26919774

commit 6c760df2f6a9f5d1ce7051be6acfeebc26919774
Author: nick <nick@chromium.org>
Date: Wed Aug 02 01:41:42 2017

[TaskManager] Allow a Task to mutate its PID after creation

Prior to this change, WebContentsTaskProvider would defer task creation if the
PID was not known at RenderFrameCreated time. However, for a newly-cloned
WebContents, no event ever occurred to re-create the Task once the process
finished launching. This was  bug 738169 .

Fix the problem by never deferring task_manager::Task creation; instead, create
Tasks for all live, current RenderFrameHosts. If the PID is not known immediately,
allow it to be set on the task at a later time (when RenderProcessHost::IsReady()
becomes true).

Because the TaskManagerImpl was written with the expectation that Tasks have
a constant PID, PID mutation is implemented in terms of temporarily removing the
task, mutating the PID, and re-adding it.

In WebContentsTaskProvider, we no longer need to implement RenderProcessGone.

In TaskGroup, don't waste time collecting stats for a null process ID.

Tests are added exercising this bug, and its subframe equivalent. Test coverage
is added for the PID==0 case, and we ensure that the "Memory" column is
populated with something nonzero.

BUG= 738169 

Review-Url: https://codereview.chromium.org/2961423002
Cr-Commit-Position: refs/heads/master@{#491180}

[modify] https://crrev.com/6c760df2f6a9f5d1ce7051be6acfeebc26919774/chrome/browser/task_manager/providers/task.cc
[modify] https://crrev.com/6c760df2f6a9f5d1ce7051be6acfeebc26919774/chrome/browser/task_manager/providers/task.h
[modify] https://crrev.com/6c760df2f6a9f5d1ce7051be6acfeebc26919774/chrome/browser/task_manager/providers/task_provider.cc
[modify] https://crrev.com/6c760df2f6a9f5d1ce7051be6acfeebc26919774/chrome/browser/task_manager/providers/task_provider.h
[modify] https://crrev.com/6c760df2f6a9f5d1ce7051be6acfeebc26919774/chrome/browser/task_manager/providers/web_contents/web_contents_task_provider.cc
[modify] https://crrev.com/6c760df2f6a9f5d1ce7051be6acfeebc26919774/chrome/browser/task_manager/sampling/task_group.cc
[modify] https://crrev.com/6c760df2f6a9f5d1ce7051be6acfeebc26919774/chrome/browser/task_manager/task_manager_browsertest.cc
[modify] https://crrev.com/6c760df2f6a9f5d1ce7051be6acfeebc26919774/chrome/browser/task_manager/task_manager_browsertest_util.cc
[modify] https://crrev.com/6c760df2f6a9f5d1ce7051be6acfeebc26919774/chrome/browser/task_manager/task_manager_browsertest_util.h
[modify] https://crrev.com/6c760df2f6a9f5d1ce7051be6acfeebc26919774/chrome/browser/task_manager/task_manager_tester.cc