Add methods to enable configuration of ResourceConstraints based on limits derived at runtime.

Adds ConfigureResourceConstraintsForCurrentPlatform and SetDefaultResourceConstraintsForCurrentPlatform which configure the heap based on the available physical memory, rather than hard-coding by platform as previous.  This change also adds OS::TotalPhysicalMemory to platform.h.

BUG=292928
R=danno@chromium.org, hpayer@chromium.org

Review URL: https://codereview.chromium.org/24269003

Patch from Ross McIlroy <rmcilroy@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16964 ce2b1a6d-e550-0410-aec6-3dcde31c8c00