Use the full range of OOM priority for renderer processes in OomPriorityManager.

Previously we would increase the priority in steps based on the number of tabs, but there could be fewer renderers than tabs.

BUG=none

Review URL: https://codereview.chromium.org/145323007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246759 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246759
Revert 242135 "Added merge session request throttle for XHR requ..."

> Added merge session request throttle for XHR requests.
>
> BUG=308318, 309131, 308681
> TEST=manual, added MergeSessionTest.MergeSessionThrottle
>
> Review URL: https://codereview.chromium.org/118733002

TBR=zelidrag@chromium.org

Review URL: https://codereview.chromium.org/145013013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246760 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246760