This occurs in the wasm scenario described in the referenced bug.
DecodeWasmModule collects statistics. Blink inserts a CreateHistogramCallback that
can't instantiate a histogram on non-joinable threads. Turns out, DecodeWasmModule
is scheduled on such a thread, now that we have async compilation.

This fix pre-initializes histograms when the callback is applied, which is assumed to
be in a context that can carry out the instantiation. In Blink, this happens on the main
thread.

BUG=chromium:709684

Review-Url: https://codereview.chromium.org/2812543002
Cr-Commit-Position: refs/heads/master@{#44522}