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

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 97eb6726ca36455f5b87284c80ecc8c85f8c875e..501d80a95b2bff7380d65e42385d8be66319924b 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -310,6 +310,14 @@ config("compiler") {
     cflags += [ "-fcolor-diagnostics" ]
   }
 
+  # Makes builds independent of absolute file path.
+  # Currently disabled for nacl since its toolchain lacks this flag (too old).
+  # TODO(zforman): Once nacl's toolchain is updated, remove check.
+  if (is_clang && !is_nacl) {
+    absolute_path = rebase_path("//.")
+    cflags += [ "-fdebug-prefix-map=$absolute_path=." ]
+  }
+
   # C++11 compiler flags setup.
   # ---------------------------
   if (is_linux || is_android || (is_nacl && is_clang)) {