Fix ContainerNode::getElementById() with empty ID differently

We previously tried to fix this in
Ia65024efef9e5ef54d9db233590a17b372282ed1, but it appears that we were
overly optimistic. This commit puts in place a more conversative fix
that does not change anything other than
ContainerNode::getElementById(). Note that this is a different function
from Document::getElementById().

Instead of filtering out the empty ID in AttributeChanged(), put an
empty-string special case in ContainerNode::getElementById(), similar to
what is currently done for Document::getElementById(). This fixes the
original bug without implicating other parts of Blink.

Fixed: 1068580
Change-Id: Ib54bbc444babd9cbae4409cdecc09640bcb757c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2146787Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Timothy Gu <timothygu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758647}

third_party/blink/renderer/core/dom/container_node.cc

@@ -1551,6 +1551,12 @@ RadioNodeList* ContainerNode::GetRadioNodeList(const AtomicString& name,
 }
 
 Element* ContainerNode::getElementById(const AtomicString& id) const {
+  // According to https://dom.spec.whatwg.org/#concept-id, empty IDs are
+  // treated as equivalent to the lack of an id attribute.
+  if (id.IsEmpty()) {
+    return nullptr;
+  }
+
   if (IsInTreeScope()) {
     // Fast path if we are in a tree scope: call getElementById() on tree scope
     // and check if the matching element is in our subtree.

third_party/blink/web_tests/external/wpt/dom/nodes/DocumentFragment-getElementById-expected.txt

-This is a testharness.js-based test.
-PASS The method must exist
-PASS It must return null when there are no matches
-PASS It must return the first element when there are matches
-FAIL Empty string ID values assert_equals: Even if there is an element with an empty-string ID attribute, it must not be returned expected null but got Element node <div id=""></div>
-PASS It must return the first element when there are matches, using a template
-Harness: the test ran to completion.
-