[chromium] Add WebDiscardableMemory to platform
https://bugs.webkit.org/show_bug.cgi?id=107183

Reviewed by James Robinson.

WebDiscardable provides a wrapper around memory that can be
relcaimed by the underlying kernel when it is under heavy memory
pressure. This will be used in a followup patch in the Chromium
image decoding store for decoded/resized images.

* Platform.gypi:
* chromium/public/Platform.h:
(WebKit):
(Platform):
(WebKit::Platform::allocateDiscardableMemory):
* chromium/public/WebDiscardableMemory.h: Added.
(WebKit):
(WebDiscardableMemory):
(WebKit::WebDiscardableMemory::~WebDiscardableMemory):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@140499 268f45cc-cd09-0410-ab3c-d52691b4dbfc