Give chromiumcontent.dll a .def file

We use a wrapper around gyp-win-tool that notices when
chromiumcontent.dll is being linked. We dump out the exports of all the
.lib files being used to create chromiumcontent.dll and put them in a
.def file, which we use when linking chromiumcontent.dll. This causes
chromiumcontent.dll to export all the exported functions from its
constituent static libraries.
Get things closer to building on Windows

We now export symbols from gles2_c_lib.

We now only export ANGLE symbols when building libEGL.dll and
libGLESv2.dll.

We also do some tricky things with ICU symbols. What we'd really like is
to mark symbols from icudt.dll as __declspec(dllimport) and all other
symbols as __declspec(dllexport), but there's no easy way to do that
without patching ICU. So instead we we mark *all* symbols as
__declspec(dllexport) when building any static libraries, so that our
gyp-win-tool wrapper will notice the symbols and try to export them. But
when building shared libraries or executables we mark the symbols with
nothing at all. This will work just fine since the symbols will be
compiled into the DLL directly.