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: svn://svn.chromium.org/blink/trunk@176061 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/wtf/BitVector.h

@@ -194,7 +194,7 @@ private:
         return bits | (static_cast<uintptr_t>(1) << maxInlineBits());
     }
 
-    class OutOfLineBits {
+    class WTF_EXPORT OutOfLineBits {
     public:
         size_t numBits() const { return m_numBits; }
         size_t numWords() const { return (m_numBits + bitsInPointer() - 1) / bitsInPointer(); }