Add easy way to ensure the next allocation does GC.

Added a class called ScopedHeapFill which changes the bytes allocated
counter to be equal to the growth limit. This causes the next
allocation to do a GC and possibly generate an OOM error. This is
useful for tests which need GC to happen at specific point.

Change-Id: Ibd8f3d5928b58534c5165ba7c296980002aa2c28