Pass -no-canonical-prefixes to clang by default.

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 6a495cc2a82b8b2500a5c7c0c5398b7389c8f0a3..8e39ab77859eb7f10dd2bc7a526071c5d5664e52 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -437,6 +437,17 @@ config("compiler") {
     cflags += [ "-fdebug-prefix-map=$absolute_path=." ]
   }
 
+  # Tells the compiler not to use absolute paths when passing the default
+  # paths to the tools it invokes. We don't want this because we don't
+  # really need it and it can mess up the goma cache entries. It would
+  # be nice if it was on by default in clang, but it isn't.
+  #
+  # TODO(thakis): Figure out if this should be the default, and expose in
+  # clang-cl if not.
+  if (is_clang && !is_win) {

[shinyak, 2017/04/26 01:47:36]

It looks pnacl-clang does not recognize -no-canonical-prefixes, we need to have
"&& !is_nacl" for now.

+    cflags += [ "-no-canonical-prefixes" ]
+  }
+
   # C++11 compiler flags setup.
   # ---------------------------
   if (is_linux || is_android || (is_nacl && is_clang)) {