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On Android we explicitly allow the OS to kill Chrome's background
renderers when under memory pressure and we don't try to control the
number of renderers ourselves.

The process limit logic in content causes process sharing
between unrelated tabs when the number of renderer process hosts
(not the number of actual live processes) is too high. Because on
Android the system adjusts the number of actual live processes for us,
we don't want to limit the number of process hosts or to ever share
renderers between unrelated tabs.

This patch:
- disables the renderer process host limit on Android. If not
  overridden, ShouldTryToUseExistingProcessHost() will always return
  false.
- drops the logic that sets the renderer limit based on the number of
  declared renderer services

BUG=325842

Review URL: https://codereview.chromium.org/356453003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284095 0039d316-1c4b-4281-b951-d872f2087c98