This adds two new UMA histograms to give us more insight into Wasm
memory allocation.

The first records the result of every attempt to to allocate a Wasm
backing store. This will let us know things like how often we
explicitly trigger a GC, or how often we hit our address space limit.

The second records how many megabytes of address space Wasm reserves.
A sample is added every time the number either increases or decreases.
This metric will give us a sense of how many outstanding Wasm memories
there are in typical usage.

Change-Id: I38c1bc1ad915c26b6cda3c373ededdd395193a4c
Reviewed-on: https://chromium-review.googlesource.com/1024646
Commit-Queue: Eric Holk <eholk@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52767}