Add libc++ __tree to ubsan blacklist.

libc++ __tree is not ubsan clean. It floods compile and run steps
with "runtime error: downcast" and "runtime error: upcast" errors,
making ubsan much less usable.
This is a KI with libc++: https://llvm.org/bugs/show_bug.cgi?id=19302

Review-Url: https://codereview.chromium.org/2109663003
Cr-Commit-Position: refs/heads/master@{#402902}

tools/ubsan/blacklist.txt

@@ -69,3 +69,9 @@ fun:*re2*RegexpD*
 #############################################################################
 # Harmless float division by zero.
 fun:*RendererFrameManager*CullUnlockedFrames*
+
+#############################################################################
+# libc++ __tree and map are not UBSAN clean
+# https://llvm.org/bugs/show_bug.cgi?id=19302
+src:*/third_party/libc\+\+/trunk/include/__tree
+src:*/third_party/libc\+\+/trunk/include/map