Bug 1090398 - Workaround for scrollinfo layers not yet having event regions set up correctly. r=

Until bug 1082594 is fixed, the area occupied by inactive scrollable subframes
don't get added to their layer's dispatch-to-content region. Therefore the APZ
code doesn't know what area corresponds to the subframe, even though it knows
there is a subframe because of the scrollinfo layer. This change inserts a small
workaround to deal with the scrollinfo layers specially, so that the composition
bounds from the scrollinfo layer is used to obtain the area. Once bug 1082594
is fixed this can probably be removed.
Bug 1090398 - Distinguish the various outcomes of APZ hit testing more explicitly. r=

Previously we output of APZ hit testing was an APZC instance and an "in
overscroll" flag. Now we need to also distinguish between the hit region and the
dispatch-to-content region for an APZC, so this patch introduces an enum that
makes this more explicit.
Bug 1090398 - Require APZC target confirmation before input blocks are ready for handling. r=

For proper hit-testing we need to deal with situations where the initial touch
input falls on a dispatch-to-content region of an APZC, and the main thread
later tells us that it actually hit another APZC. This patch allows input blocks
to have an "unconfirmed" APZC target for the initially-found target that can
then be replaced with a different target. Input blocks without a confirmed
target are not ready for processing.
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