Enable DumpAccessibilityTree tests to use cross-process iframes.

content/test/data/accessibility/readme.txt

 DumpAccessibilityTreeTest and DumpAccessibilityEventsTest Notes:
 
 Both sets of tests use a similar format for files.
 
 DumpAccessibilityTree tests load an HTML file, wait for it to load, then
 dump the accessibility tree in the "blink" format (the internal data),
 and again in a platform-specific format.
 
 The test output is a compact text representation of the accessibility tree
 for that format, and it should be familiar if you're familiar with the
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 Normally the system waits for the document to finish loading before dumping
 the accessibility tree.
 
 Occasionally you may need to write a test that makes some changes to the
 document before it runs the test. In that case you can use a special
 @WAIT-FOR: directive. It should be in an HTML comment, just like
 @ALLOW-WIN: directives. The WAIT-FOR directive just specifies a text substring
 that should be present in the dump when the document is ready. The system
 will keep blocking until that text appears.
 
+You can add as many @WAIT-FOR: directives as you want, the test won't finish
+until all strings appear.
+
 To skip dumping a particular element, make its accessible name equal to
 @NO_DUMP, for example <div aria-label="@NO_DUMP"></div>.
 
 To skip dumping all children of a particular element, make its accessible
 name equal to @NO_CHILDREN_DUMP.
 
+To load an iframe from a different site, forcing it into a different process,
+use @CROSS-SITE: in the url, for example:
+  <iframe src="@CROSS-SITE:iframe.html"></iframe>
+
 Generating expectations and rebaselining:
 
 If you want to populate the expectation file directly rather than typing it
 or copying-and-pasting it, first make sure the file exists (it can be empty),
 then run the test with the --generate-accessibility-test-expectations
 argument, for example:
 
   out/Debug/content_browsertests \
     --generate-accessibility-test-expectations
     --gtest_filter="DumpAccessibilityTreeTest.AccessibilityA"
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 To write a test for accessibility events, your document must contain a
 JavaScript function called go(). This function will be called when the document
 is loaded (or when the @WAIT_FOR directive passes), and any subsequent
 events will be dumped. Filters apply to events just like in tree dumps.
 
 After calling go(), the system asks the page to generate a sentinel
 accessibility event - one you're unlikely to generate in your test. It uses
 that event to know when to "stop" dumping events. There isn't currently a
 way to test events that occur after some delay, just ones that happen as
 a direct result of calling go().