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In Chrome Apps, some platform APIs can only be called from the background
page (e.g. reloading a chrome.app.AppWindow).  Likewise, some chrome API's
must be initiated by user interaction, which can only be called from the
foreground.

This CL base.IPC that provides helper functions to invoke methods on
different pages using chrome.runtime.sendMessage.

BUG=452317

Review URL: https://codereview.chromium.org/877993002

Cr-Commit-Position: refs/heads/master@{#313614}