Disabled two flaky subtests of SignalThreadTest.
Disabled two flaky subtests of SignalThreadTest.

BUG=webrtc:5953

Review-Url: https://codereview.webrtc.org/2029523002
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#13019}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a8aa9076391a328e54b97673b60baa03f8d9795e

Alkalyne/webrtctrunk,jchavanton/webrtc,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,jchavanton/webrtc,jchavanton/webrtc,jchavanton/webrtc,jchavanton/webrtc,Alkalyne/webrtctrunk,jchavanton/webrtc,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,jchavanton/webrtc,Alkalyne/webrtctrunk