Avoid crash after ve.dm.Surface.purgeHistory().

The selection property is never null; it is initialized to Range(0,0).
If it is set to null in purgeHistory(), the next call to
ve.dm.Surface.change() will crash.

Change-Id: Ia45c0ba26291e8ad09c445fdf2323710b5ab409f
Make ve.dm.Surface.{start,stop}HistoryTracking idempotent.

Add a little robustness, guaranteeing that we don't end up with multiple
history tracking tasks running, leaking one, or try to clear a non-running
interval.

Change-Id: I41db2d6fefc7f45f150aa14ecefc648760ad6200
Merge "Make ve.dm.Surface.{start,stop}HistoryTracking idempotent."
Don't render <meta>/<link>/<style> tags in GeneratedContentNode

There is no good reason why we'd render these tags, and their presence
causes Chrome to crash while cutting across them.

Bug: 50043
Change-Id: I611e3907cf20fa27dbef89ea941d0b787a44ba4f
Merge "Don't render <meta>/<link>/<style> tags in GeneratedContentNode"