When exporting a HistoryItem, always export the main frame's item.

This was regressed in blink r162228, where it switched to exporting the item
for whichever frame requested the HistoryItem.

BUG=334293
TEST=WebFrameTest.ExportHistoryItemFromChildFrame

Review URL: https://codereview.chromium.org/133683007

git-svn-id: svn://svn.chromium.org/blink/trunk@165155 bbb929c8-8fbe-4397-9dbb-9b2b20218538