Template Expressions: Use a double escape for commas in i18nPolymer

The single escape does not work correctly for the case of replacements
within an HTML template string in JS, and the double escape works
correctly both for the template string in JS case and the normal HTML
replacement case.

Discovered while migrating the extensions page to use Polymer 3,
specifically the i18nPolymer replacements in shortcut_input.html,
which contain commas for Chrome OS.

Bug: 1004967
Change-Id: Ifb7a743cc2048942294a1cb93f73871191ffa254
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1826279Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700428}

ui/base/template_expressions.cc

@@ -78,7 +78,7 @@ std::string PolymerParameterEscape(const std::string& in_string) {
         out.append(""");
         break;
       case ',':
-        out.append("\\,");
+        out.append("\\\\,");
         break;
       default:
         out += c;

ui/base/template_expressions_unittest.cc

@@ -78,12 +78,12 @@ TEST(TemplateExpressionsTest, ReplaceTemplateExpressionsPolymerMixed) {
   static TemplateReplacements substitutions;
   substitutions["punctuationSample"] = "a\"b'c<d>e&f,g";
   substitutions["htmlSample"] = "<div>hello</div>";
-  EXPECT_EQ("a&quot;b\\'c<d>e&f\\,g",
+  EXPECT_EQ("a&quot;b\\'c<d>e&f\\\\,g",
             ReplaceTemplateExpressions("$i18nPolymer{punctuationSample}",
                                        substitutions));
   EXPECT_EQ("<div>hello</div>", ReplaceTemplateExpressions(
                                     "$i18nPolymer{htmlSample}", substitutions));
-  EXPECT_EQ("multiple: <div>hello</div>, a&quot;b\\'c<d>e&f\\,g.",
+  EXPECT_EQ("multiple: <div>hello</div>, a&quot;b\\'c<d>e&f\\\\,g.",
             ReplaceTemplateExpressions("multiple: $i18nPolymer{htmlSample}, "
                                        "$i18nPolymer{punctuationSample}.",
                                        substitutions));