Create and export sandbox_static.lib for Windows clients

sandbox.lib isn't usable out of the box because it expects base's
symbols to be decorated with __declspec(dllexport). So we create a copy
of it that expects them to be decorated with __declspec(dllimport),
as they will be when used in a client application.
Export base_static.lib for Windows clients
Fix exporting of WebKit symbols

We had a duplicate target_definitions dictionary.
Generate libchromiumcontent-symbols.zip on Windows

PDB files required for building go into libchromiumcontent.zip. PDB
files only required for debugging go into
libchromiumcontent-symbols.zip.
Use the DLL version of the CRT

This is required so embedding apps can pass types like std::string into
chromiumcontent.dll.

The DLL CRT seems to require exceptions to be enabled. This matches what
Chromium does for component builds.
Export libGLESv2.dll on Windows
Export icudt.dll for Windows clients