Add function to check what HTML Element interface corresponds to a given tag name.

third_party/WebKit/Source/core/BUILD.gn

Index: third_party/WebKit/Source/core/BUILD.gn
diff --git a/third_party/WebKit/Source/core/BUILD.gn b/third_party/WebKit/Source/core/BUILD.gn
index 85a7c9410bed2e583425a99865d7db4a358fd8d2..58df744e78eccde47e97c84801a837e21b400a32 100644
--- a/third_party/WebKit/Source/core/BUILD.gn
+++ b/third_party/WebKit/Source/core/BUILD.gn
@@ -495,6 +495,7 @@ process_in_files("make_core_generated_html_element_type_helpers") {
   in_files = [ "html/HTMLTagNames.in" ]
   other_inputs = make_element_type_helpers_files
   outputs = [
+    "$blink_core_output_dir/HTMLElementTypeHelpers.cpp",
     "$blink_core_output_dir/HTMLElementTypeHelpers.h",
   ]
 }