Fix MIDL cross-compilation for ARM64 Win32 targets

A change in dependencies means that a component with a MIDL dependency
(third_party/iacccessible2) is now compiled for the x64 host and arm64.
This caused problems because the host win_clang_x64 generated files
typically live in out/<t>/win_clang_x64/gen/third_party/iaccessible2
whereas the arm64 files live in out/<t>/gen/third_party/iaccessible2,
meaning that (when the path is rebased) the midl action tried to copy
files from a missing win_clang_x64 directory in iaccessible2. Using the
root_gen_dir instead fixes this depth mismatch.

Bug: 1105878
Change-Id: I9403b0024c394b339168e47275d23240b55c3574
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2302054Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Richard Townsend <richard.townsend@arm.com>
Cr-Commit-Position: refs/heads/master@{#790105}

build/toolchain/win/midl.gni

@@ -55,7 +55,7 @@ template("midl") {
   } else {
     # midl.py expects 'gen' to be replaced with 'midl'.
     generated_dir = rebase_path("//third_party/win_build_output") + "/midl/" +
-                    rebase_path(rebase_path(out_dir, root_build_dir), "gen")
+                    rebase_path(out_dir, root_gen_dir)
   }
 
   if (defined(invoker.dynamic_guid)) {