C++ local var initialization always tricks me. Previously, Layout
didn't have a constructor, which meant that defining it on the stack
left mAdvance uninitialized. This was not an issue when we were doing
"new Layout()", since that invokes zero-initialization, but was an
issue for the skipCache path that was allocating layout on stack by
just "Layout l" instead of "Layout l = Layout()". To avoid surprises,
add a constructors that clears everything.
Also adds reset() method to reset the layout for reuse.
Change-Id: I3e02f00da9dd7d360abe13f63c310f6882292d0a
df03550a40