Fix prerendering disabled crash on android

When prerendering is disabled, we might get NULL for the PrerenderManager.  We should
NULL check instead of assuming it is a valid pointer.

BUG=

Review URL: https://codereview.chromium.org/74513002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235632 0039d316-1c4b-4281-b951-d872f2087c98
Blink roll 162160:162164

http://build.chromium.org/f/chromium/perf/dashboard/ui/changelog_blink.html?url=/trunk&range=162161:162164&mode=html
TBR=
BUG=

Review URL: https://codereview.chromium.org/74713002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235633 0039d316-1c4b-4281-b951-d872f2087c98
Update .DEPS.git

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235634 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235634