<third_party/WebKit>: Prevent cl.exe crash on Windows

cl.exe does not like long relative include paths. In shadow builds that result
in large relative paths, some of the IDL binding include paths are too long
and often completely incorrect, specifically for IDL files that are generated
and are not sourced in the Blink repository. This patch resolves that problem
and makes the paths minimal for generated IDL bindings, preventing the cl.exe
fatal C1001 crash.

Change-Id: I912e2c8d2c4a6de79693c63ff5313958360cb42b
Reviewed-by: Michael Bruning <michael.bruning@digia.com>