Fix -Winconsistent-missing-override in CrashReportUploadThread

Fix -Winconsistent-missing-override in CrashReportUploadThread

Chromium builds with a newer clang than the Crashpad buildbot, and it
reports:

../../../handler/crash_report_upload_thread.cc:148:16: error: 'ThreadMain' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]
  virtual void ThreadMain() {
               ^
../../../util/thread/thread.h:46:16: note: overridden virtual function is here
  virtual void ThreadMain() = 0;
               ^
1 error generated.

R=scottmg@chromium.org

Committed: https://chromium.googlesource.com/crashpad/crashpad/+/899821d4c791bdedcc0882312f90a67320332bac

[Mark Mentovai, 2015-08-19 21:17:17]

mark@chromium.org changed reviewers:
+ scottmg@chromium.org

[Mark Mentovai, 2015-08-19 21:17:17]

While I was in here, I waffled about eliminating
internal::CrashReportUploadHelperThread and just making CrashReportUploadThread
inherit from Thread directly. But it seemed like a bad idea to expose
Thread::Start() and Thread::Stop() to anyone that happens to have a pointer to a
CrashReportUploadThread.

[scottmg, 2015-08-19 23:25:34]

Thanks, lgtm.

I waffled on the inheritance too, but decided containment was better also.

[Mark Mentovai, 2015-08-20 02:41:51]

Committed patchset #1 (id:1) manually as
899821d4c791bdedcc0882312f90a67320332bac (presubmit successful).