Just a simple cleanup CL. TryAllocateBackingStore function is only needed in
wasm-memory.cc, so this makes that stronger by putting it in an anonymous
namespace. Additionally, the whole function is moved to the top of the file.

No functional change.

R=gdeepti@chromium.org

Change-Id: I0c5ea07c1ab81f3083eb75f0a6177c503fc827b5
Reviewed-on: https://chromium-review.googlesource.com/985023Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52290}