Bug 952505 - Update bundled libpng to version 1.6.9. r=jmuizelaar
Bug 936340 - Implement navigator.sendBeacon. r=sicking
Bug 967927 - Fix Clang build warning in gstreamer. r=eflores
Bug 970348 - Add background-blend-mode reftests corresponding to the csswg test plan. r=roc, r=heycam

* 5.3.1: Blending between the background layers and the background color for an element with background-blend-mode. Most of section 5.3.1 is covered by other tests. Add the missing tests of the 5.3.1 section of the test plan.
- Test 9: SVG image and background color (SVG as data:uri)
- Test 10: Animated gif image and background color
- Test 12: background-blend-mode on element with 3D transform

* 5.3.2: Background layers do not blend with content outside the background (or behind the element).
- Implementation of tests 2 and 3. Test 1 is already covered by background-blending-isolation.html.

* 5.3.6: background-blend-mode for an element with background-position

* 5.3.7: background-blend-mode for an element with background-size

* 5.3.8: background-blend-mode for an element with background-repeat

* 5.3.9: background-blend-mode for an element with background-clip

* 5.3.10: background-blend-mode for an element with background-origin

* 5.3.11: background-blend-mode for an element with background-attachement

* Add extra tests for the background-blend-mode CSS property: Add some invalid values to the background-blend-mode property definition so that it can benefit from existing invalid property tests.