StringView could use a function to strip leading/trailing characters without allocation

StringView could use a function to strip leading/trailing characters without allocation
https://bugs.webkit.org/show_bug.cgi?id=175757

Reviewed by Darin Adler.

Source/WTF:

There are many places in WebCore/WebKit that we call functions like,
WebCore::stripLeadingAndTrailingHTMLSpaces,  or String::stripWhiteSpace() only to use
the allocated String as a temporary for either another transformation or a comparison.
Now that we have StringView, we can avoid that extra allocation, by having returning a
StringView  substring in these scenarios.

For instance, the check (from ScriptElement.cpp:287):

if (!stripLeadingAndTrailingHTMLSpaces(sourceURL).isEmpty()) {
    ...
}

currently allocates a string just to make this check. With a new
stripLeadingAndTrailingHTMLSpaces such as:

StringView stripLeadingAndTrailingHTMLSpaces(StringView stringView)
{
    return stringView.stripLeadingAndTrailingMatchedCharacters([] (auto c) {
        return isHTMLSpace(c);
    });
}

We could instead have exact same code from ScriptElement.cpp now avoid an allocation.

* wtf/text/StringView.h:
(WTF::StringView::stripLeadingAndTrailingMatchedCharacters):

Tools:

* TestWebKitAPI/Tests/WTF/StringView.cpp:
Add tests for StringView::stripLeadingAndTrailingMatchedCharacters().

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@220982 268f45cc-cd09-0410-ab3c-d52691b4dbfc