Reverts fdca07 and makes it possible to force absolute paths.

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 60954b3dcc064e606e2db5f8ed1ac909c60b413f..dde34c1eac64bd041b2d48ea0097d50efbde2b5e 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -307,6 +307,15 @@ config("compiler") {
     cflags += [ "-fcolor-diagnostics" ]
   }
 
+  # Makes builds independent of absolute file path.
+  # clang-cl (used if is_win) doesn't expose this flag.
+  # 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_win && !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)) {