ax_dump_tree: NULL value of AXSubrole is prefixed by const_

Bug: 1124366
Change-Id: I2ab34d8aeee10e7598fd0eb05ce9cb294e835d4d
AX-Relnotes: n/a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2412452Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Alexander Surkov <asurkov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#808830}

content/browser/accessibility/accessibility_tree_formatter_mac.mm

@@ -444,9 +444,14 @@ std::string AccessibilityTreeFormatterMac::ProcessTreeForOutput(
   std::string subrole_attr = SysNSStringToUTF8(NSAccessibilitySubroleAttribute);
   value = dict.FindStringPath(subrole_attr);
   if (value) {
-    WriteAttribute(false,
-                   StringPrintf("%s=%s", subrole_attr.c_str(), value->c_str()),
+    if (*value == kNULLValue) {
+      WriteAttribute(false, StringPrintf("%s=NULL", subrole_attr.c_str()),
                      &line);
+    } else {
+      WriteAttribute(
+          false, StringPrintf("%s=%s", subrole_attr.c_str(), value->c_str()),
+          &line);
+    }
   }
 
   // Expose all other attributes.