clang: Makes builds with clang less dependent on absolute file path

build/config/sysroot.gni

Index: build/config/sysroot.gni
diff --git a/build/config/sysroot.gni b/build/config/sysroot.gni
index 29a2b6117a30cb77da72b2ff24773f8d925d4b13..0a7373e5b912ed75b02e0c2189cb4d237521551a 100644
--- a/build/config/sysroot.gni
+++ b/build/config/sysroot.gni
@@ -37,13 +37,17 @@ if (current_toolchain == default_toolchain && target_sysroot != "") {
   # By default build against a sysroot image downloaded from Cloud Storage
   # during gclient runhooks.
   if (current_cpu == "x64") {
-    sysroot = rebase_path("//build/linux/debian_wheezy_amd64-sysroot")
+    sysroot =
+        rebase_path("//build/linux/debian_wheezy_amd64-sysroot", root_build_dir)
   } else if (current_cpu == "x86") {
-    sysroot = rebase_path("//build/linux/debian_wheezy_i386-sysroot")
+    sysroot =
+        rebase_path("//build/linux/debian_wheezy_i386-sysroot", root_build_dir)
   } else if (current_cpu == "mipsel") {
-    sysroot = rebase_path("//build/linux/debian_wheezy_mips-sysroot")
+    sysroot =
+        rebase_path("//build/linux/debian_wheezy_mips-sysroot", root_build_dir)
   } else if (current_cpu == "arm") {
-    sysroot = rebase_path("//build/linux/debian_wheezy_arm-sysroot")
+    sysroot =
+        rebase_path("//build/linux/debian_wheezy_arm-sysroot", root_build_dir)
   } else {
     # Any other builds don't use a sysroot.
     sysroot = ""
@@ -63,7 +67,9 @@ if (current_toolchain == default_toolchain && target_sysroot != "") {
       _script_arch = "amd64"
     }
     assert(
-        exec_script("//build/dir_exists.py", [ sysroot ], "string") == "True",
+        exec_script("//build/dir_exists.py",
+                    [ rebase_path(sysroot, "", root_build_dir) ],
+                    "string") == "True",
         "Missing sysroot ($sysroot). To fix, run: build/linux/sysroot_scripts/install-sysroot.py --arch=$_script_arch")
   }
 } else if (is_mac) {