Prevent exception when key is pressed with no selection in list view
Prevent exception when key is pressed with no selection in list view

The picture ops list view extends list_view's functionality to support
deselecting if a selected element is clicked twice. If a key is pressed
in this case, an exception would be thrown because list_view assumes
there is a selected element.

This patch just checks that this.selectedElement is defined in the key
event listener.

TBR=nduca

Review URL: https://codereview.appspot.com/11643043
catapult-project/catapult-csm,catapult-project/catapult,catapult-project/catapult,danbeam/catapult,scottmcmaster/catapult,catapult-project/catapult-csm,zeptonaut/catapult,catapult-project/catapult-csm,sahiljain/catapult,0x90sled/catapult,catapult-project/catapult,modulexcite/catapult,catapult-project/catapult-csm,dstockwell/catapult,catapult-project/catapult-csm,benschmaus/catapult,catapult-project/catapult,zeptonaut/catapult,sahiljain/catapult,SummerLW/Perf-Insight-Report,0x90sled/catapult,benschmaus/catapult,benschmaus/catapult,vmpstr/trace-viewer,SummerLW/Perf-Insight-Report,sahiljain/catapult,dstockwell/catapult,benschmaus/catapult,catapult-project/catapult,catapult-project/catapult,sahiljain/catapult,zeptonaut/catapult,vmpstr/trace-viewer,danbeam/catapult,modulexcite/catapult,scottmcmaster/catapult,scottmcmaster/catapult,benschmaus/catapult,danbeam/catapult,danbeam/catapult,benschmaus/catapult,dstockwell/catapult,dstockwell/catapult,sahiljain/catapult,catapult-project/catapult,SummerLW/Perf-Insight-Report,catapult-project/catapult-csm,benschmaus/catapult,SummerLW/Perf-Insight-Report,vmpstr/trace-viewer,catapult-project/catapult-csm,sahiljain/catapult,SummerLW/Perf-Insight-Report,SummerLW/Perf-Insight-Report,0x90sled/catapult,modulexcite/catapult