DumpAccTree testsuite: convert utf16 to utf8 in FormatAccessibilityTree

Bug: 1115489
Change-Id: I0eec812a35bab1d49183279c87e31d87e1216a8e
AX-Relnotes: n/a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363840
Commit-Queue: Alexander Surkov <[email protected]>
Reviewed-by: Dominic Mazzoni <[email protected]>
Reviewed-by: Lei Zhang <[email protected]>
Reviewed-by: Alex Moshchuk <[email protected]>
Cr-Commit-Position: refs/heads/master@{#803827}
diff --git a/content/browser/web_contents/web_contents_impl.cc b/content/browser/web_contents/web_contents_impl.cc
index 64f3342..cc23bbd 100644
--- a/content/browser/web_contents/web_contents_impl.cc
+++ b/content/browser/web_contents/web_contents_impl.cc
@@ -3856,7 +3856,7 @@
   });
 }
 
-base::string16 WebContentsImpl::DumpAccessibilityTree(
+std::string WebContentsImpl::DumpAccessibilityTree(
     bool internal,
     std::vector<AccessibilityTreeFormatter::PropertyFilter> property_filters) {
   auto* ax_mgr = GetOrCreateRootBrowserAccessibilityManager();