Makes dynamic sizing of core-dropdown possible

Currently, the size of the dropdown is set in stone the first time the user views it. If the contents of the dropdown change in content or size later, the dropdown will not adjust. This is because measure() does not remove the width/height which is set from the results of measure() in previous runs, thus the size is always the "same".

In addition to this PR, I have pushed a fork of the current production tag (0.4.2) along with a patch onto:
https://github.com/rezonant/core-dropdown/tree/0.4.2_measurepatch. 

Reproduction
1. core-dropdown is opened
2. core-dropdown may be closed or left open
3. content changes, causing a new size
4. core-dropdown's overlay size is now incorrect until the widget is reloaded