Add HTML referrerpolicy attribute for area element
The `referrerpolicy` attribute is implemented for the <area> element via
inheritance from HTMLAnchorElement, so this CL enables it by adding it
to the IDL and adds tests.
BUG=
490608
Committed:
https://crrev.com/6569abdd1213a68eef95e0ad1ee63df465f2f2aa
Cr-Commit-Position: refs/heads/master@{#374101}