Link client apps against base_static.lib

This is required to get base::win::PEImage, which is required by sandboxing
code.
Use __declspec(dllimport) for functions from chromiumcontent.dll
Fix sandbox linking errors

* vendor/libchromiumcontent b48a089...5f1bf90 (7):
  > Create a copy of sandbox.lib that can be used in client apps
  > Try to fix linker errors with sandbox.lib
  > Get rid of duplicate target_conditions definitions
  > Let's see if we can get a very simple case working
  > Fix typo
  > Try to make setting WEBKIT_DLL actually work
  > Export base_static.lib for Windows clients