DO NOT MERGE Fix flakiness in WebChromeClientTest#testWindows

 Bug 6460200
 childWindow was referenced in the fireOpen function and called
 setTimeout("childWindow.close(), 2000). If gc occured between
 the call to setTimeout and the callback, childWindow would have
 been collected and the reference lost. This change adds a reference
 outside of the function so that it cannot be collected.

Change-Id: Id3717104c3a1a0b7da26bfb0a7f939e88d9ed582