Pepper: Refactor trusted plugin properties.

As part of the "trusted plugin refactor", we need to change properties on the
plugin's embed element to be set through an interface instead of PPAPI
scripting. This is one of a series of changes to carve up functionality in the
trusted plugin and move it elsewhere.

After this change, ScriptablePlugin doesn't seem very useful, but I'm not
completely sure if it (and PPP_Instance_Private) can be removed. I'll look at
that separately.

BUG=239656

Review URL: https://codereview.chromium.org/84773004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@237604 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 237604