This change defines a common config for clients of v8 that
need to define V8_USE_EXTERNAL_STARTUP_DATA in order for
client code to be compiled correctly.

R=brettw@chromium.org, jochen@chromium.org
BUG=541791
LOG=n

Review URL: https://codereview.chromium.org/1415773003

Cr-Commit-Position: refs/heads/master@{#31427}