Bug 1113010 (part 0) - Remove impossible path in AddFrameConstruct(). r=dholbert.

Infallible new ensures that |item| is always non-null. And even if it didn't,
AppendItem() dereferences |item| before this code is reached.

--HG--
extra : rebase_source : 9abb8704ba03f455d6b77c5735fcb6cde4f8fef8
Bug 1113010 (part 1) - Shrink FrameConstructionItem by reordering some fields. r=dholbert.

This reduces its size from 176 bytes to 168 bytes on 64-bit platforms.

--HG--
extra : rebase_source : 89b4f1c6e618eb1ab75989162c26308753a59d56
Bug 1113010 (part 2) - Shrink FrameConstructionItem by using bitfields. r=dholbert.

This reduces its size from 168 bytes to 160 bytes on 64-bit platforms.

--HG--
extra : rebase_source : de9bb482ccbdb85efa79f82083fa8f35bb629078