Hello,
as I can see in:
AtomicString Document::convertLocalName(const AtomicString& name) {
  return isHTMLDocument() ? name.lower() : name;
}

we can change to:
AtomicString Document::convertLocalName(const AtomicString& name) {
  return isHTMLDocument() ? name.lowerASCII() : name;
}
And tests will be ok (also it fix some fails in Document-createAttribute.html tests).

But after this fix we have another problem:
Fail	createElement("ınput") in HTML document	assert_equals: tagName expected "ıNPUT" but got "INPUT"

What do you think about it?

I guess the first thing to work out is: What does the spec say to do? Is the test correct? And, what do other browsers do?

Using BrowserStack -
Safari 10 - everything passes.
Edge 14 - fails all of the İnput and marK tests (9 tests) due to an InvalidCharacterError. It also fails ெfoo tests and some more tests (15 tests) due to not throwing.
Internet Explorer 11 - same as Edge 14. Funny - Internet Explorer 6 did not throw that error. :)
Firefox 50 - everything passes.
Chrome - 3 failures -
createElement("marK") in HTML document - localName expected "marK" but got "mark"
createElement("İnput") in HTML document - localName expected "İnput" but got "i̇nput"
createElement("ınput") in HTML document - tagName expected "ıNPUT" but got "INPUT"

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1337a0b763dc552f6eeadb17df348429b34a3501

commit 1337a0b763dc552f6eeadb17df348429b34a3501
Author: ratsunny <ratsunny@gmail.com>
Date: Wed Mar 01 22:08:41 2017

Use the correct case converter for |localName| and |tagName|

Per spec[1], document.createElement should set |localName|
to ASCII lowercase when context object is an HTML document.

Now the AtomicString::lower() was used,
but it converts characters more than ASCII into lowercase.
So we should use AtomicString::lowerASCII() instead.

Besides, per spec[2], Element.tagName should be set to ASCII
uppercase when context object is in HTML namespace and its node
document is an HTML document.

Now we are using (Atomic)String::upper() due to lack of
ASCII uppercase support. So (Atomic)String::upperASCII() is
added to solve this issue, and AtomicString::upper() is removed
since it's not used anymore.

Tests were changed accordingly

[1]: https://dom.spec.whatwg.org/#dom-document-createelement
[2]: https://dom.spec.whatwg.org/#dom-element-tagname

BUG= 685012 

Review-Url: https://codereview.chromium.org/2723053002
Cr-Commit-Position: refs/heads/master@{#454059}

[delete] https://crrev.com/494a3b618ecf62e96f14c195a0d2234b20db785c/third_party/WebKit/LayoutTests/external/wpt/dom/nodes/Document-createElement-expected.txt
[modify] https://crrev.com/1337a0b763dc552f6eeadb17df348429b34a3501/third_party/WebKit/LayoutTests/external/wpt/dom/nodes/Document-getElementsByTagName-expected.txt
[modify] https://crrev.com/1337a0b763dc552f6eeadb17df348429b34a3501/third_party/WebKit/LayoutTests/external/wpt/dom/nodes/Element-getElementsByTagName-expected.txt
[modify] https://crrev.com/1337a0b763dc552f6eeadb17df348429b34a3501/third_party/WebKit/LayoutTests/external/wpt/dom/nodes/case-expected.txt
[modify] https://crrev.com/1337a0b763dc552f6eeadb17df348429b34a3501/third_party/WebKit/Source/core/dom/Document.cpp
[modify] https://crrev.com/1337a0b763dc552f6eeadb17df348429b34a3501/third_party/WebKit/Source/core/dom/QualifiedName.cpp
[modify] https://crrev.com/1337a0b763dc552f6eeadb17df348429b34a3501/third_party/WebKit/Source/core/html/HTMLElement.cpp
[modify] https://crrev.com/1337a0b763dc552f6eeadb17df348429b34a3501/third_party/WebKit/Source/wtf/text/AtomicString.cpp
[modify] https://crrev.com/1337a0b763dc552f6eeadb17df348429b34a3501/third_party/WebKit/Source/wtf/text/AtomicString.h
[modify] https://crrev.com/1337a0b763dc552f6eeadb17df348429b34a3501/third_party/WebKit/Source/wtf/text/StringImpl.cpp
[modify] https://crrev.com/1337a0b763dc552f6eeadb17df348429b34a3501/third_party/WebKit/Source/wtf/text/StringImpl.h
[modify] https://crrev.com/1337a0b763dc552f6eeadb17df348429b34a3501/third_party/WebKit/Source/wtf/text/StringImplTest.cpp
[modify] https://crrev.com/1337a0b763dc552f6eeadb17df348429b34a3501/third_party/WebKit/Source/wtf/text/WTFString.cpp
[modify] https://crrev.com/1337a0b763dc552f6eeadb17df348429b34a3501/third_party/WebKit/Source/wtf/text/WTFString.h