Bug 919144. Part 1: Rename identifiers from 'active scroll(ed) root' to 'animated geometry root'. r=mattwoodrow
Also sneaks in a couple of other refactorings:

--HG--
extra : rebase_source : 8b0e0729f26231d98ed069b54dd4f7c1e51b10b5
Bug 919144. Part 2: Force creation of a layer for every viewport with a displayport. r=mattwoodrow
This ensures that for all fixed-pos content attached to a viewport with a
displayport, the viewport has a layer and the fixed-pos content's layer will be
a direct child of the viewport's layer.

--HG--
extra : rebase_source : da35bacad5518b94cb0b92a198d4f607cede4ac9
Bug 919144. part 3: The displayport out parameters to GetDisplayPort and GetCriticalDisplayPort are optional, so make them optional. r=mattwoodrow

--HG--
extra : rebase_source : 487f895279982c4b74fcd720c5c97381e46036d8
Bug 919144. Part 4: Create nsLayoutUtils::IsFixedPosFrameInDisplayPort. r=mats

--HG--
extra : rebase_source : a7b7179efd91838ec7205f7109182c78814d33c5
Bug 919144. Part 5: Add ThebesLayerData::mReferenceFrame. r=mattwoodrow

--HG--
extra : rebase_source : f569d1712be8e43aeff31ca2f045a4b87bbd1de6
Bug 919144. Part 6: nsDisplayListBuilder::MarkOutOfFlowFrameForDisplay should call IsFixedPosFrameInDisplayPort and make fixed-pos frames intersect their overflow-rect with the viewport's displayport to get their dirty rect. r=mattwoodrow
This prevents insanely large fixed-pos elements from getting an over-huge
dirty rect and building an enormous display list.
This is also a step towards getting rid of nsDisplayListBuilder::mHasDisplayPort,
which is good because the new code works on any subdocument that has a displayport.

--HG--
extra : rebase_source : e853022dd2ce85af318f235f8851d9dd8b5001ac
Bug 919144. Part 7: Make fixed-pos frames with displayports animated geometry roots, and make FrameLayerBuilder responsible for setting fixed-pos layer parameters instead of nsDisplayFixedPosition. r=mattwoodrow
This patch also makes FrameLayerBuilder responsible for setting the visible region
of a fixed-pos layer to the displayport intersected with the draw-region of
the layer. This will let us remove some displaylist code in later patches.
We add special handling to GetAnimatedGeometryRootFor for nsDisplayScrollLayers,
to match what nsDisplayList::IsFixedItem does.

--HG--
extra : rebase_source : f9258243fe666d5dee33c13a88e596bbed8e2c1c
Bug 919144. Part 8: Delete lots of code that's no longer needed. r=mattwoodrow
I never liked nsDisplayListBuilder::IsFixedItem anyway, its behavior and
naming was confusing. Also, it assumed there's only one viewport displayport,
on the rootmost document, and that's a bad assumption in general.
We're able to remove calls to ForceVisiblityForFixedItem because FrameLayerBuilder
is now responsible for inflating the visible region of the fixed-pos layers.

--HG--
extra : rebase_source : e93d8e25d2f5c7f3e167e47203d961e6b4293b81
Bug 919144. Part 9: Move FrameLayerBuilder::ContainerParameters to mozilla::ContainerLayerParameters. r=mattwoodrow
This is to avoid having to #include FrameLayerBuilder.h in nsLayoutUtils.h in the next patch.

--HG--
extra : rebase_source : 5f3c3ae2785267949766ae094414781133d4ce44
Bug 919144. Part 10: Fold nsDisplayFixedPosition into nsDisplayStickyPosition. r=mattwoodrow

--HG--
extra : rebase_source : 597e2a178762509421a32d53be6242cd588c7995