Added docs on accessibility tests

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
-accessibility protocol on that platform, but it's not in a standardized
-format - it's just a text dump, meant to be compared to expected output.
-
-The exact output can be filtered so it only dumps the specific attributes
-you care about for a specific test.
-
-One the output has been generated, it compares the output to an expectation
-file in the same directory. If an expectation file for that test for that
-platform is present, it must match exactly or the test fails. If no
-expectation file is present, the test passes. Most tests don't have
-expectations on all platforms.
-
-DumpAccessibilityEvent tests use a similar format but dump events fired after
-the document finishes loading. See more on this below.
-
-Compiling and running the tests:
-ninja -C out/Debug content_browsertests
-out/Debug/content_browsertests --gtest_filter="DumpAccessibility*"
-
-Files used:
-* foo.html -- a file to be tested
-* foo-expected-android.txt -- expected Android AccessibilityNodeInfo output
-* foo-expected-auralinux.txt -- expected Linux ATK output
-* foo-expected-blink.txt -- representation of internal accessibility tree
-* foo-expected-mac.txt -- expected Mac NSAccessibility output
-* foo-expected-win.txt -- expected Win IAccessible/IAccessible2 output
-
-Format for expected files:
-* Blank lines and lines beginning with # are ignored
-* Skipped files: if first line of file begins with #<skip then the
-  test passes. This can be used to indicate desired output with a link
-  to a bug, or as a way to temporarily disable a test during refactoring.
-* Use 2 plus signs for indent to show hierarchy
-
-Filters:
-* By default only some attributes of nodes in the accessibility tree, or
-  events fired (when running DumpAccessibilityEvents) are output.
-  This is to keep the tests robust and not prone to failure when unrelated
-  changes affect the accessibility tree in unimportant ways.
-* Filters contained in the HTML file can be used to control what is output.
-  They can appear anywhere but typically they're in an HTML comment block,
-  and must be one per line.
-* Filters are platform-specific:
-    @WIN-
-    @MAC-
-    @BLINK-
-    @ANDROID-
-    @AURALINUX-
-* To dump all attributes while writing or debugging a test, add this filter:
-    @WIN-ALLOW:*
-  (and similarly for other platforms).
-* Once you know what you want to output, you can use filters to match the
-  attributes and attribute values you want included in the output. An
-  ALLOW filter means to include the attribute, and a DENY filter means to
-  exclude it. Filters can contain simple wildcards ('*') only, they're not
-  regular expressions. Examples:
-  - @WIN-ALLOW:name* - this will output the name attribute on Windows
-  - @WIN-ALLOW:name='Foo' - this will only output the name attribute if it
-      exactly matches 'Foo'.
-  - @WIN-DENY:name='X* - this will skip outputting any name that begins with
-      the letter X.
-* By default empty attributes are skipped. To output the value an attribute
-  even if it's empty, use @WIN-ALLOW-EMPTY:name, for example, and similarly
-  for other platforms.
-
-Advanced:
-
-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/HOSTNAME/ in the url, for example:
-  <iframe src="cross-site/1.com/accessibility/html/frame.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"
-
-This will replace the -expected-*.txt file with the current output. It's
-a great way to rebaseline a bunch of tests after making a change. Please
-manually check the diff, of course!
-
-Adding a new test:
-
-If you are adding a new test file remember to add a corresponding test case in
-content/browser/accessibility/dump_accessibility_events_browsertest.cc
-or
-content/browser/accessibility/dump_accessibility_tree_browsertest.cc
-
-More details on DumpAccessibilityEvents tests:
-
-These tests are similar to DumpAccessibilityTree tests in that they first
-load an HTML document, then dump something, then compare the output to
-an expectation file. The difference is that what's dumped is accessibility
-events that are fired.
-
-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().