Remove redundant Document::getElementById
https://bugs.webkit.org/show_bug.cgi?id=122813

Reviewed by Andreas Kling.

Merge https://chromium.googlesource.com/chromium/blink/+/4e8f1c5316415614b84370c602beae4a1008299f

This function simply calls virtual TreeScope::getElementById and Document inherits from TreeScope.

* WebCore.exp.in:
* dom/Document.cpp:
* dom/Document.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@157444 268f45cc-cd09-0410-ab3c-d52691b4dbfc