Pass setScriptableObjectProperty calls on to Pepper again

We previously removed the ability for Pepper plugins to intercept
properties set on the plugin element as we thought that this
private feature wasn't used anywhere. It turns out that this feature
is still used by the deprecated gTalk plugin. This patch adds back
the ability for the plugin to intercept the call.

Note that we are missing the ability (in V8) to detect whether
the plugin has actually intercepted the call the avoid passing
the SetProperty request onto the DOM element. But that turns
out to be inconsequential for the gTalk plugin.

BUG=414842

Review URL: https://codereview.chromium.org/640073005

git-svn-id: svn://svn.chromium.org/blink/trunk@183492 bbb929c8-8fbe-4397-9dbb-9b2b20218538