Introduce PlatformHandle

In UNIX-based systems any system resource we deal with is an
"int" (a file descriptor). On Windows the situation is more
complex because there are (At least) two types of resources:
1. HANDLE objects, coming from long-standing winapi.
2. int resources, coming from the more modern posix-compat
   layer (e.g. _open, _close, etc).
This CL introduces PlatformHandle to deal with types that
are still int on UNIX but a HANDLE on Windows.


Bug: 174454879
Change-Id: Ief3ea5d6a79952c1f1577cd0515476818bbf3d92
diff --git a/BUILD b/BUILD
index 991e2fb..8a1fa06 100644
--- a/BUILD
+++ b/BUILD
@@ -272,6 +272,7 @@
         "include/perfetto/base/export.h",
         "include/perfetto/base/flat_set.h",
         "include/perfetto/base/logging.h",
+        "include/perfetto/base/platform_handle.h",
         "include/perfetto/base/proc_utils.h",
         "include/perfetto/base/status.h",
         "include/perfetto/base/task_runner.h",