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Generalize usage of discardable memory in the renderer process to use
higher-level discardable memory primitives (DiscardableMemoryAllocator
and DiscardableMemory).

Implement a factory method to create a discardable memory allocator
implementation in renderer context depending on platform support.

Note that usage of MADV_FREE DiscardableMemory is gated by a feature
flag. Discardable memory should behave exactly as before this
change if the feature flag is not enabled.

Binary-Size: Unused ForTest method not being removed by linker
Bug: 1014513
Change-Id: I94afd2347a6ecbe254cd2d1a3ad667a6624293ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1850758
Commit-Queue: Gordon Guan <gordonguan@google.com>
Reviewed-by: Peng Huang <penghuang@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Benoit L <lizeb@chromium.org>
Reviewed-by: Egor Pasko <pasko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713010}