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Shares some of the build rules from ChromeVox, but with no extension tests or
Closure compilation rules yet. We can add those later as needed.

If you run Chrome with the --enable-experimental-accessibility-features
flag, then toggling the Select-to-speak checkbox in the accessibility
preferences loads or unloads this component extension, as verified by the
presence of the (empty) background script in chrome://inspect

BUG=593887
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation
TBR=rdevlin.cronin

Review-Url: https://codereview.chromium.org/2216433004
Cr-Commit-Position: refs/heads/master@{#419318}