Remove transient and unnecessary update density message.

On a page load, the (viewport) metadata settings for the main frame are
reset. Reloading a page with viewport meta tag specifying target density
dpi will result in 2 back to back update zoom density messages to be
sent from WebViewCore to UI thread.

In setupViewport(), remove update density messages in flight. If the new
density is indeed different from WebView's default scale, queue another
message.

Bug: 6509512
Change-Id: I195e40277bf1bcccb0e61d84ed84db1e34556243