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-Original-Commit-Position: refs/heads/master@{#317651}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 92d69f7733f4028cf8b3d5b4b97a31173e687594

win/dbghelp_xp/README.chromium

@@ -0,0 +1,2 @@
+This dbghelp.dll is the redistributable version from the Windows 7 SDK, the
+last one to work on Windows XP.