GC_SHRINK is a fairly new type of GC that does more aggressive cleanups than a
normal GC. This patch makes the browser run the GC in this mode when under
memory pressure, or when the user pushes the Minimize Memory Usage button when
on the about:memory page.
The new boolean argument to SetDebugModeWhenPossible is needed because if we allow sync-disable of debug mode we seem to crash when loading pages with Firebug active.
