Avoid variable/type shadowing in blink/dom

In an effort to reduce or even ban variable shadowing, this renames
a variable to avoid such shadowing. I'm interested in prohibiting
shadowing because I think it might prevent potential jumbo problems.

The exact warnings this avoids are:
third_party/blink/renderer/core/dom/element.cc:3940:35: error: declaration shadows a local variable [-Werror,-Wshadow]
        else if (const Attribute* attribute =
                                  ^
third_party/blink/renderer/core/dom/element.cc:3938:30: note: previous declaration is here
        if (const Attribute* attribute = attributes.Find(xml_names::kLangAttr))
                             ^
third_party/blink/renderer/core/dom/node.cc:2113:17: error: declaration shadows a local variable [-Werror,-Wshadow]
    const Node* node = chain[index - 1];
                ^
third_party/blink/renderer/core/dom/node.cc:2107:15: note: previous declaration is here
  const Node* node = this;
              ^
third_party/blink/renderer/core/dom/range.cc:529:36: error: declaration shadows a type alias in namespace 'blink' [-Werror,-Wshadow]
  typedef HeapVector<Member<Node>> NodeVector;
                                   ^
third_party/blink/renderer/core/dom/container_node.h:84:7: note: previous declaration is here
using NodeVector = HeapVector<Member<Node>, kInitialNodeVectorSize>;
      ^
third_party/blink/renderer/core/dom/range.cc:750:36: error: declaration shadows a type alias in namespace 'blink' [-Werror,-Wshadow]
  typedef HeapVector<Member<Node>> NodeVector;
                                   ^
third_party/blink/renderer/core/dom/container_node.h:84:7: note: previous declaration is here
using NodeVector = HeapVector<Member<Node>, kInitialNodeVectorSize>;
      ^

Bug: 925310

Change-Id: I8bc037dd1cf54baf5f6fe1ed93b1ead391313e8a
Reviewed-on: https://chromium-review.googlesource.com/c/1478876
Reviewed-by: Fredrik Söderquist <fs@opera.com>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Auto-Submit: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#634659}