Walking heaps from GetProcessHeaps is flaky by design, failing on XP

There are two possible race conditions in walking the heaps returned
by GetProcessHeaps and on Windows XP we hit one of these race conditions,
manifested in a crash, on 42 of the last 200 base unittests. Disabling
the test entirely until a better fix is found (or perhaps forever).

The unreliability of the underlying code is acceptable for tracing,
although perhaps not acceptable for tracing on XP given the failure rate.

TBR=sebmarchand@chromium.org,primiano@chromium.org
BUG=487291

Review URL: https://codereview.chromium.org/1140943006

Cr-Commit-Position: refs/heads/master@{#330423}

base/trace_event/winheap_dump_provider_win_unittest.cc

@@ -13,7 +13,9 @@
 namespace base {
 namespace trace_event {
 
-TEST(WinHeapDumpProviderTest, OnMemoryDump) {
+// This test is flaky on XP and is not guaranteed reliable on any version of
+// Windows. https://crbug.com/487291
+TEST(WinHeapDumpProviderTest, DISABLED_OnMemoryDump) {
   ProcessMemoryDump pmd(make_scoped_refptr(new MemoryDumpSessionState()));
 
   WinHeapDumpProvider* winheap_dump_provider =