Implemented declarative redirects to transparent image and empty document.

chrome/common/extensions/docs/static/webRequest.html

 <div id="pageData-name" class="pageData">Web Requests</div>
 
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 <p id="classSummary">
 Use the <code>chrome.webRequest</code> module to intercept, block,
 or modify requests in-flight and to observe and analyze traffic.
 </p>
 
 <h2 id="manifest">Manifest</h2>
 <p>You must declare the "webRequest" permission in the <a
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   requests, this means that the status line and response headers are
   available. This event is informational and handled asynchronously. It does not
   allow modifying or cancelling the request.</dd>
   <dt><code>onCompleted</code></dt>
   <dd>Fires when a request has been processed successfully.</dd>
   <dt><code>onErrorOccurred</code></dt>
   <dd>Fires when a request could not be processed successfully.</dd>
 </dl>
 The web request API guarantees that for each request either
 <code>onCompleted</code> or <code>onErrorOccurred</code> is fired as the final
-event.
+event with one exception: If a request is redirected to a <code>data://</code>
+URL, <code>onBeforeRedirect</code> is the last reported event.
 </p>
 
 <p id="life_cycle_footnote">(*) Note that the web request API presents an
 abstraction of the network stack to the extension. Internally, one URL request
 can be split into several HTTP requests (for example to fetch individual byte
 ranges from a large file) or can be handled by the network stack without
 communicating with the network. For this reason, the API does not provide the
 final HTTP headers that are sent to the network. For example, all headers that
 are related to caching are invisible to the extension.</p>
 
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     }
     return {requestHeaders: details.requestHeaders};
   },
   {urls: ["&lt;all_urls&gt;"]},
   ["blocking", "requestHeaders"]);
 </pre>
 
 <p> For more example code, see the <a href="samples.html#webrequest">web request
 samples</a>.</p>
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