Bug 921928 - Make <iframe mozbrower remote> a preferred target when fluffing r=roc

In bug 921928, the user places a call and then tries to do other actions
(calls, SMS, contacts, ...) with actions very close to the attention
screen. When trying to do those other actions, event fluffing is
prioritizing the attention screen rather than user apps.
---
 layout/base/PositionedEventTargeting.cpp           |  10 ++
 layout/base/tests/Makefile.in                      |   2 +
 .../bug921928_event_target_iframe_apps_oop.html    |   8 +
 .../base/tests/test_event_target_iframe_oop.html   | 178 +++++++++++++++++++++
 4 files changed, 198 insertions(+)
 create mode 100644 layout/base/tests/bug921928_event_target_iframe_apps_oop.html
 create mode 100644 layout/base/tests/test_event_target_iframe_oop.html