Add placeholder text to hint_: method

This CL ensures that placeholder text will be read out
by ChromeVox.

placeholder text.

Test: manual, follow bug repro steps. ChromeVox will read
Test: TODO write tast test
Bug: b/130369098
Change-Id: I491811d9ba6e951ef365eed137768a12a8150222
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1855621Reviewed-by: Sara Kato <sarakato@chromium.org>
Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Sara Kato <sarakato@chromium.org>
Auto-Submit: Sara Kato <sarakato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705854}

chrome/browser/resources/chromeos/chromevox/cvox2/background/output.js

@@ -2153,6 +2153,9 @@ Output.prototype = {
             hintProperties);
     }
 
+    if (node.placeholder)
+      this.append_(buff, node.placeholder);
+
     if (AutomationPredicate.checkable(node))
       this.format_(
           node, '@hint_checkable', buff, ruleStr, undefined, hintProperties);