This CL introduces an import section that names functions to be imported
as well as a CallImport bytecode to call imports from this table.

R=binji@chromium.org,bradnelson@chromium.org
LOG=Y
BUG=chromium:575167

Review URL: https://codereview.chromium.org/1709653002

Cr-Commit-Position: refs/heads/master@{#34157}