Add dbghelp.dll that works on XP, and include in base.isolate

This fixes the hang that caused the addition of the early out in
stack_trace_win.cc.

Removing that in turn fixes symbolization of stack traces on XP bots.

R=cpu@chromium.org
BUG=460506

Review URL: https://codereview.chromium.org/943933005

Cr-Commit-Position: refs/heads/master@{#317651}

base/base.isolate

@@ -30,6 +30,14 @@
         ],
       },
     }],
+    ['OS=="win"', {
+      # Required for base/stack_trace_win.cc to symbolize correctly.
+      'variables': {
+        'files': [
+          '../build/win/dbghelp_xp/dbghelp.dll',
+        ],
+      },
+    }],
     ['OS=="win" and asan==1 and component=="shared_library"', {
       'variables': {
         'files': [

base/debug/stack_trace_win.cc

@@ -152,10 +152,6 @@ class SymbolContext {
 
     init_error_ = ERROR_SUCCESS;
 
-    // Work around a mysterious hang on Windows XP.
-    if (base::win::GetVersion() < base::win::VERSION_VISTA)
-      return;
-
     // When transferring the binaries e.g. between bots, path put
     // into the executable will get off. To still retrieve symbols correctly,
     // add the directory of the executable to symbol search path.

build/win/dbghelp_xp/README.chromium

+This dbghelp.dll is the redistributable version from the Windows 7 SDK, the
+last one to work on Windows XP.