[Android] Add regression tests for JS wrappers usage in Java Bridge

[Android] Add regression tests for JS wrappers usage in Java Bridge

Add two new tests verifying that each context uses its own JS wrapper
object, and that populating JS wrappers in one context doesn't break
wrappers in other contexts.

Also fixed existing testInjectedObjectPresentInChildFrame test, which
apparently was doing a wrong thing:

  1) iframe isn't a blank tag, so it must have a proper closing tag;

  2) iframe's 'onload' event is executed in the context of the main page,
     so the test was in fact interacting with the injected object of
     the main page.

BUG=372914
TBR=benm@chromium.org

Review URL: https://codereview.chromium.org/333863003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@277503 0039d316-1c4b-4281-b951-d872f2087c98