Fix libc++ dependency for linux installer

Without this dependency, "gn gen" complains that libc++ isn't generated by any dependencies of the targets when building an ASAN build with is_chrome_branded=true on Linux.

Here are the gn args I used:

is_chrome_branded = true
is_asan = true
is_component_build = false
is_debug = false
is_lsan = true
symbol_level = 1

Here are the errors:

Generating files...
ERROR Input to targets not generated by a dependency.
The file:
  //out/GN_Asan/lib/libc++.so
is listed as an input or source for the targets:
  //chrome/installer/linux:stable_deb
  //chrome/installer/linux:stable_rpm
  //chrome/installer/linux:beta_deb
  //chrome/installer/linux:beta_rpm
  //chrome/installer/linux:unstable_deb
  //chrome/installer/linux:unstable_rpm
  //chrome/installer/linux:trunk_deb
  //chrome/installer/linux:trunk_rpm
  //chrome/installer/linux:asan_deb
  //chrome/installer/linux:asan_rpm
but no targets in the build generate that file.

Review-Url: https://codereview.chromium.org/2478973002
Cr-Commit-Position: refs/heads/master@{#431137}
diff --git a/chrome/installer/linux/BUILD.gn b/chrome/installer/linux/BUILD.gn
index c9e0e78..4e8bff35 100644
--- a/chrome/installer/linux/BUILD.gn
+++ b/chrome/installer/linux/BUILD.gn
@@ -201,6 +201,9 @@
   if (!is_chromeos) {
     public_deps += [ ":rpm_packaging_files" ]
   }
+  if (use_custom_libcxx) {
+    public_deps += [ "//buildtools/third_party/libc++" ]
+  }
 }
 
 # Creates .deb and .rpm (RPM for non-ChromeOS only) installer packages.
@@ -247,8 +250,8 @@
     ]
   }
 
-  if (is_asan) {
-    packaging_files_binaries += [ "$root_out_dir/lib/libc++.so" ]
+  if (use_custom_libcxx) {
+    packaging_files_binaries += [ "$root_out_dir/libc++.so" ]
   }
 
   deb_target_name = "${target_name}_deb"