Add sdk_root property to windows_sdk module

Bug: 926631
Change-Id: Ie2ce79dd5f2cb75ef5160dda5ff5d8dd497beead
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1518793Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>

recipes/recipe_modules/windows_sdk/api.py

@@ -32,6 +32,10 @@ class WindowsSDKApi(recipe_api.RecipeApi):
       enabled (bool): Whether the SDK should be used or not.
       target_arch (str): 'x86' or 'x64'.
 
+    Yields:
+      If enabled, yields Path to the root of the 'win_sdk' directory in the SDK
+      to be used by recipes that need additional setup.
+
     Raises:
         StepFailure or InfraFailure.
     """
@@ -41,7 +45,7 @@ class WindowsSDKApi(recipe_api.RecipeApi):
           version or self._sdk_properties['version'])
       try:
         with self.m.context(**self._sdk_env(sdk_dir, target_arch)):
-          yield
+          yield sdk_dir.join('win_sdk')
       finally:
         # cl.exe automatically starts background mspdbsrv.exe daemon which
         # needs to be manually stopped so Swarming can tidy up after itself.