Fix segv in ui::Clipboard::WriteObjects() in clipboard_aurax11.cc

Check that the vectors received from the renderer are not empty before
accesssing specific elements.

BUG=341510

Review URL: https://codereview.chromium.org/144003023

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249877 0039d316-1c4b-4281-b951-d872f2087c98
Blink roll 166732:166741

http://build.chromium.org/f/chromium/perf/dashboard/ui/changelog_blink.html?url=/trunk&range=166733:166741&mode=html
TBR=
BUG=

Review URL: https://codereview.chromium.org/135513007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249878 0039d316-1c4b-4281-b951-d872f2087c98
Update .DEPS.git

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249879 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249879