This commit changes wasm_exec.js to not depend on the existence of
performance.timeOrigin. The field is not yet supported on all
browsers, e.g. it is unavailable on Safari.

Change-Id: I6cd3834376c1c55424c29166fde1219f0d4d338f
Reviewed-on: https://go-review.googlesource.com/118617Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>