tools/gn: do not break bootstrap process on Windows

malloc_dump_provider.cc should be built only on Linux or Mac.

BUG=568437
TBR=brettw@chromium.org

Review URL: https://codereview.chromium.org/1521063002

Cr-Original-Commit-Position: refs/heads/master@{#364841}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 4514a7974f1acd2cd696aa3e1833c2e7be970668