Track page protection status in the shadow memory, and only use BlockInfoFromShadow if necessary.

Performance improvements measured via an instrumented M41 Chrome.

BEFORE
HeapAlloc  :  2506
HeapReAlloc: 24592
HeapFree   :  5201
HeapSize   :  1235

AFTER
HeapAlloc  :  2286 (9% improvement)
HeapReAlloc: 22269 (10% improvement)
HeapFree   :  4767 (8% improvement)
HeapSize   :   829 (33% improvement)

BUG=
R=sebmarchand@chromium.org

Review URL: https://codereview.appspot.com/181280043