Add duplicate libcxx source repo paths.

Context: go/unnesting-buildtools-lib
We are moving these source files buildtools/third_party/lib*/trunk -> third_party/libcxx/lib*/trunk.

rollout plan is:
1. This Cl
2. in chromium/src merge https://chromium-review.googlesource.com/c/chromium/src/+/4666325
3. remove old buildtools/third_party/lib*/trunk paths in DEPS

work tracked here: http://shortn/_BqvAmLrzlr

Bug: chromium:1458042
Change-Id: Ibe35a63f75a838350a97520dd108516f1cabba88
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/142520
Reviewed-by: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Joanna Wang <jojwang@google.com>
diff --git a/DEPS b/DEPS
index 18fb0fc..334db24 100644
--- a/DEPS
+++ b/DEPS
@@ -85,6 +85,8 @@
     'condition': 'dawn_standalone and host_os == "win"',
   },
 
+  # TODO(chromium:1458042): Remove these paths, when chromium builds files
+  # have moved to third_party/libcxx/lib*/trunk paths.
   'buildtools/third_party/libc++/trunk': {
     'url': '{chromium_git}/external/github.com/llvm/llvm-project/libcxx.git@0e4617cf8c09a8e2b6704a51a8a0a9560715cf70',
     'condition': 'dawn_standalone',
@@ -95,6 +97,16 @@
     'condition': 'dawn_standalone',
   },
 
+  'third_party/libcxx/libc++/trunk': {
+    'url': '{chromium_git}/external/github.com/llvm/llvm-project/libcxx.git@0e4617cf8c09a8e2b6704a51a8a0a9560715cf70',
+    'condition': 'dawn_standalone',
+  },
+
+  'third_party/libcxx/libc++abi/trunk': {
+    'url': '{chromium_git}/external/github.com/llvm/llvm-project/libcxxabi.git@307bd163607c315d46103ebe1d68aab44bf93986',
+    'condition': 'dawn_standalone',
+  },
+
   'tools/clang': {
     'url': '{chromium_git}/chromium/src/tools/clang@effd9257d456f2d42e9e22fa4f37a24d8cf0b5b5',
     'condition': 'dawn_standalone',