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loadable_modules are quite commonly used in tests and for plugins, yet
it is barely mentioned in GN's docs.

Add it to the cookbook so people porting their projects away from gyp
know that the construct also exists in GN.

R=brettw@chromium.org,dpranke@chromium.org

Review-Url: https://codereview.chromium.org/2276943005
Cr-Commit-Position: refs/heads/master@{#414458}