Update VE core submodule to master (d6d6db4)

New changes:
378bc4c Polyfill Node.prototype.normalize in IE
d54cd30 Call onTargetInputChange when the link inspector is ready
00f7a00 Only undo if an initial annotation was applied
5d39686 Make filibuster code debug only
fdda60a Track selectionBefore in staging transactions

Change-Id: I2a70d599838b3575c5b0a63ffc60d023b04a8e80
Merge "Update VE core submodule to master (d6d6db4)"