Give BlobData an explicit, private operator=.

clang/win needs this, but MSVS2014 will need it too. See
http://llvm.org/PR20782 for some discussion.

Fixes

E:\b\depot_tools\win_toolchain\vs2013_files\win8sdk\bin\..\..\VC\include\xutility(2045,10) :  error(clang): object of type 'blink::BlobDataItem' cannot be assigned because its copy assignment operator is implicitly deleted
                *_Dest = *_First;
                       ^
E:\b\depot_tools\win_toolchain\vs2013_files\win8sdk\bin\..\..\VC\include\xutility(2065,10) :  note(clang): in instantiation of function template specialization 'std::_Copy_impl<const blink::BlobDataItem *, blink::BlobDataItem *>' requested here
        return (_Copy_impl(_First, _Last,
                ^
E:\b\depot_tools\win_toolchain\vs2013_files\win8sdk\bin\..\..\VC\include\xutility(2096,2) :  note(clang): in instantiation of function template specialization 'std::_Copy_impl<const blink::BlobDataItem *, blink::BlobDataItem *>' requested here
        _Copy_impl(_First, _Last,
        ^
E:\b\depot_tools\win_toolchain\vs2013_files\win8sdk\bin\..\..\VC\include\xutility(2116,10) :  note(clang): in instantiation of function template specialization 'std::_Copy_impl<const blink::BlobDataItem *, blink::BlobDataItem *>' requested here
        return (_Copy_impl(_First, _Last,
                ^
E:\b\depot_tools\win_toolchain\vs2013_files\win8sdk\bin\..\..\VC\include\xutility(2131,11) :  note(clang): in instantiation of function template specialization 'std::_Copy_impl<const blink::BlobDataItem *, blink::BlobDataItem *>' requested here
                return (_Copy_impl(_Unchecked(_First), _Unchecked(_Last),
                        ^
..\..\third_party\WebKit\Source\wtf/Vector.h(772,14) :  note(clang): in instantiation of function template specialization 'std::copy<const blink::BlobDataItem *, blink::BlobDataItem *>' requested here
        std::copy(other.begin(), other.begin() + size(), begin());

             ^
..\..\third_party\WebKit\Source\platform/blob/BlobData.h(161,23) :  note(clang): in instantiation of member function 'WTF::Vector<blink::BlobDataItem, 0, WTF::DefaultAllocator>::operator=' requested here
class PLATFORM_EXPORT BlobData {
                      ^
..\..\third_party\WebKit\Source\platform/blob/BlobData.h(134,7) :  note(clang): copy assignment operator of 'BlobDataItem' is implicitly deleted because field 'type' is of const-qualified type 'const enum (anonymous enum at ..\..\third_party\WebKit\Source\platform/blob/BlobData.h:129:11)'
    } type;
      ^

This happens because the class is marked dllexport, and dllexported classes
get their operator= exported, so it needs to be instantiated. On
non-windows, there's no dllexport so operator= is only generated when
needed (and it happens to be needed never). MSVS2013 allows this because
it ignores const on enums, so it can generate an (incorrect) operator=.

BUG=82385

Review URL: https://codereview.chromium.org/519693003

git-svn-id: svn://svn.chromium.org/blink/trunk@181107 bbb929c8-8fbe-4397-9dbb-9b2b20218538