Add WTF_EXPORT to BitVector::OutOfLineBits
Add WTF_EXPORT to BitVector::OutOfLineBits

dllexport/dllimport aren't inherited by nested classes like visibility
attributes are on Posix, and this class has some non-inline methods that
need to get exported.

For example, if ~BitVector gets inlined, we might try calling
OutOfLineBits::destroy across a dll boundry.

BUG=82385
TEST=attempt to build blink_web.dll with Clang on Windows

Review URL: https://codereview.chromium.org/338463002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@176061 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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