Improve test engine error feedback:

content/test/data/accessibility/readme.txt

Index: content/test/data/accessibility/readme.txt
diff --git a/content/test/data/accessibility/readme.txt b/content/test/data/accessibility/readme.txt
new file mode 100644
index 0000000000000000000000000000000000000000..30977822cbef69176d58ccc1b236c00ba521212b
--- /dev/null
+++ b/content/test/data/accessibility/readme.txt
@@ -0,0 +1,18 @@
+DumpAccessibilityTreeTest Notes
+
+Files used:
+* foo.html -- a file to be tested
+* foo-expected-win.txt -- expected MSAA output
+* foo-expected-mac.txt -- expected Mac accessibility 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 entire file is ignored,
+  but listed in the output as skipped
+* Use 4 spaces for indent to show hierarchy
+* MSAA states do not have a prefix, e.g. FOCUSED, not STATE_SYSTEM_FOCUSED
+* All other constants are used exactly as normally named
+* See specific examples (e.g. ul-expected-win.txt) for more details
+
+Running the tests:
+browser_tests --gtest_filter="DumpAccessibilityTreeTest*"