This patch moves the following files to platform/wtf/.

    ASCIICType.{h,cpp}
    Assertions.h (Note: Assertions.cpp is left in wtf/; see below.)
    Atomics.h
    SpinLock.h
    StringExtras.h
    text/Unicode.h
    text/icu/UnicodeIcu.h

Assertions.cpp is still in wtf/, but this is safe because no cpp files in
wtf/ and platform/wtf/ use the symbols defined in Assertions.cpp.
Assertions.cpp has dependencies to Threading.h and ThreadSpecific.h, which
cannot be moved easily, so it will be moved in a later batch.

BUG=691465

Review-Url: https://codereview.chromium.org/2741343017
Cr-Commit-Position: refs/heads/master@{#458305}