Use relative path for isysroot

pkg_config added -isystem in abspath.

Let me use relative path here for cache utilization on goma backend when building chrome in different directory.

Change-Id: Id48db3036711c07788bb370753acc7a9b217a89d
Reviewed-on: https://chromium-review.googlesource.com/954526Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@google.com>
Cr-Commit-Position: refs/heads/master@{#541738}

build/config/linux/pkg_config.gni

@@ -57,7 +57,7 @@ if (sysroot != "") {
   # Pass the sysroot if we're using one (it requires the CPU arch also).
   pkg_config_args += [
     "-s",
-    rebase_path(sysroot),
+    rebase_path(sysroot, root_build_dir),
     "-a",
     current_cpu,
   ]