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The 'shape' attribute on HTMLAreaElement is an "enumerated attribute"
[1], which means that it should be matched ASCII case-insensitively[2].

To get a proper overload of equalIgnoringASCIICase(...) that matches the
required use (AtomicString against string literal/const char*),
restructure the definition of equalIgnoringASCIICase() to do away with
the templated version in favor of a generic StringImpl* version, and
specific wrappers around that one for AtomicString/String.
Also add a specialization for comparing against char/LChar.

No effect on the specified test because:
a) Invalid/missing value default is not per spec, so the unexpected shape
is picked anyway.
But more importantly:
b) the way case-folding is implemented/specified, no non-ASCII character
is case-folded to something in the ASCII-range.

[1] https://html.spec.whatwg.org/multipage/embedded-content.html#attr-area-shape
[2] https://html.spec.whatwg.org/multipage/infrastructure.html#enumerated-attribute

TEST=fast/html/area-shape.html
BUG=578125

Review URL: https://codereview.chromium.org/1624383002

Cr-Commit-Position: refs/heads/master@{#371478}