Move files in wtf/ to platform/wtf/ (Part 9).

At this time, String related files are moved:

    HexNumber.h, MathExtras.h, RetainPtr.h, StringHasher.h,
    WTFThreadData.h (.cpp remaining), dtoa.{h,cpp}, dtoa/*

    In text/:
        ASCIIFastPath.h, AtomicString.{h,cpp}, AtomicStringCF.cpp,
        AtomicStringHash.h, AtomicStringTable.{h,cpp}, CString.{h,cpp},
        CharacterNames.h, IntegerToStringConversion.h, StringBuffer.h,
        StringBuilder.{h,cpp}, StringConcatenate.{h,cpp},
        StringHash.h, StringImpl.{h,cpp}, StringImplCF.cpp,
        StringImplMac.mm, StringMac.mm, StringOperators.h,
        StringStatics.{h,cpp}, StringToNumber.{h,cpp},
        StringView.{h,cpp}, UTF8.{h,cpp}, WTFString.{h,cpp}

BUG=691465

Review-Url: https://codereview.chromium.org/2764243002
Cr-Commit-Position: refs/heads/master@{#458980}