Revert "Use relative path for isysroot"

This reverts commit eebaaff0.

Reason for revert: Suspect it broke the informational builds, see crbug.com/820129 for details.

Bug: 820129

Original change's description:
> 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/954526
> Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
> Commit-Queue: Takuto Ikuta <tikuta@google.com>
> Cr-Commit-Position: refs/heads/master@{#541738}

TBR=tikuta@google.com,thomasanderson@chromium.org

Change-Id: I1eaf74f65f18b3c26607a230da320fae4b1f0f54
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/955677Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541838}

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, root_build_dir),
+    rebase_path(sysroot),
     "-a",
     current_cpu,
   ]