Add libc++ pretty printers to tools/gdb

Turns this:

separator = {
  <std::__1::__basic_string_common<true>> = {<No data fields>},
  members of std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >:
  static __short_mask = 1,
  static __long_mask = 1,
  __r_ = {
    <std::__1::__compressed_pair_elem<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__rep, 0, false>> = {
      __value_ = {
        {
          __l = {
            __cap_ = 2108420,
            __size_ = 0,
            __data_ = 0x0
          },
          __s = {
            {
              __size_ = 4 '\004',
              __lx = 4 '\004'
            },
            __data_ = ", ", '\000' <repeats 20 times>
          },
          __r = {
            __words = {[0] = 2108420, [1] = 0, [2] = 0}
          }
        }
      }
    },
    <std::__1::__compressed_pair_elem<std::__1::allocator<char>, 1, true>> = {
      <std::__1::allocator<char>> = {<No data fields>}, <No data fields>}, <No data fields>},
  static npos = 18446744073709551615
}

Into this:

separator = ", "

BUG=866697
R=thakis
CC=leszeks

Change-Id: I4b35799c6023743b5876aa89a2448b8f841c7751
Reviewed-on: https://chromium-review.googlesource.com/1152087Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579085}

third_party/libcxx-pretty-printers/OWNERS

+dpranke@chromium.org
+leszeks@chromium.org
+thomasanderson@chromium.org

third_party/libcxx-pretty-printers/README.chromium

+Name: libcxx-pretty-printers
+URL: https://github.com/LeszekSwirski/libcxx-pretty-printers
+Version: dc4eb17426ce62262992d7c52aa1140f9b707c37
+License: GPL v3
+License File: LICENSE
+Security Critical: no
+
+Description:
+GDB Pretty Printers for libc++ of Clang/LLVM
+
+Local patches:
+None
+
+Upgrading:
+$ git clone https://github.com/LeszekSwirski/libcxx-pretty-printers.git
+$ rm -rf .git .gitignore
+* Add README.chromium
+* Add OWNERS

third_party/libcxx-pretty-printers/README.md

+libcxx-pretty-printers
+======================
+
+GDB Pretty Printers for libc++ of Clang/LLVM

third_party/libcxx-pretty-printers/src/gdbinit

+python
+import sys 
+sys.path.insert(0, '<path_to_libcxx-pp_src_dir>') 
+from libcxx.v1.printers import register_libcxx_printers 
+register_libcxx_printers (None) 
+end

third_party/libcxx-pretty-printers/src/libcxx/__init__.py

+

third_party/libcxx-pretty-printers/src/libcxx/v1/__init__.py

+

tools/gdb/gdbinit

 # This is gdbinit for source level debugging with -fdebug-compilation-dir
-# compile option.
+# compile option or when building with libc++.
 
 python
 
 import os
+import subprocess
+import sys
 
 def get_current_debug_file_directories():
   dir = gdb.execute("show debug-file-directory", to_string=True)
@@ -20,6 +22,28 @@ def add_debug_file_directory(dir):
               to_string=True)
 
 
+libcxx_pretty_printers_loaded = False
+def load_libcxx_pretty_printers(compile_dir):
+  global libcxx_pretty_printers_loaded
+  if libcxx_pretty_printers_loaded:
+    return
+  git = subprocess.Popen(
+      ['git', '-C', compile_dir, 'rev-parse', '--show-toplevel'],
+      stdout=subprocess.PIPE,
+      stderr=subprocess.PIPE)
+  src_dir, _ = git.communicate()
+  if git.returncode:
+    return
+  libcxx_pretty_printers = os.path.join(src_dir.rstrip(), 'third_party',
+                                        'libcxx-pretty-printers', 'src')
+  if not os.path.isdir(libcxx_pretty_printers):
+    return
+  sys.path.insert(1, libcxx_pretty_printers)
+  from libcxx.v1.printers import register_libcxx_printers
+  register_libcxx_printers(None)
+  libcxx_pretty_printers_loaded = True
+
+
 def newobj_handler(event):
   compile_dir = os.path.dirname(event.new_objfile.filename)
   if not compile_dir:
@@ -33,6 +57,8 @@ def newobj_handler(event):
   # https://crbug.com/603286#c35
   add_debug_file_directory(compile_dir)
 
+  load_libcxx_pretty_printers(compile_dir)
+
 
 # Event hook for newly loaded objfiles.
 # https://sourceware.org/gdb/onlinedocs/gdb/Events-In-Python.html