Fix flaky noop build failure of crashpad

Linking crashpad (//third_party/crashpad/crashpad:util) target
into a target built for a secondary toolchain could cause noop
build failure because of an incorrect `include_dirs` directive.

The library depends on a generated buildflag, which when in a
secondary toolchain is generated to $root_gen_dir, a directory
that includes the toolchain name (except for the primary one).

The target added $root_build_dir/gen to its `include_dirs`
which is equal to $root_gen_dir for the primary toolchain, but
distinct for secondary toolchain.

Moreover, `include_dirs` define directly in a `source_set` are
placed before any `include_dirs` values inherited from configs.
This means that $root_build_dir/gen was before $root_gen_dir in
the list ($root_gen_dir is inherited from default config when
building Chromium).

The result is that building any crashpad files would result in
them trying to first include the version of the buildflag that
was generated for the primary toolchain, and if not found, using
the correct one. This was then recorded in the depfile generated
by the compiler.

This meant that it was possible for the build to be incorrect
(as the content of the buildflag may be different between the
two toolchains) and cause flaky noop failures (as the buildlag
generation for the primary toolchain and the compilation of
the source file for the secondary toolchain are unordered, but
a dependency was recorded via the depfile leading ninja to
report a dirty build).

The fix is simple, use the correct value $root_gen_dir in the
`include_dirs` directive.

Fixed: chromium:1314711
Change-Id: Icba521313e4105713e66fa576d730b00c7e74c21
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/3579401
Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>

util/BUILD.gn

@@ -581,7 +581,7 @@ crashpad_static_library("util") {
   ]
 
   if (crashpad_is_mac || crashpad_is_ios) {
-    include_dirs += [ "$root_build_dir/gen" ]
+    include_dirs += [ "$root_gen_dir" ]
     deps += [ ":mig_output" ]
   }