Introduce base::make_unique as a replacement for std::make_unique,
introduced in C++14.
Use it in the wasm code base to construct new objects and wrap them in
a unique_ptr, making the code more compact and readable.

R=ahaas@chromium.org
BUG=v8:6474

Change-Id: I2b9c800edc456021b057f1e69d3c144889b1c9f4
Reviewed-on: https://chromium-review.googlesource.com/529167Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45822}