Mark conformance2/textures/canvas_sub_rectangle/* flaky on Linux NVIDIA.

These tests are still flaky when virtualized GL contexts are used in
the command buffer. So far no luck reproducing these failures locally.

BUG=694359
TBR=zmo@chromium.org
NOTRY=true
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2853133002
Cr-Commit-Position: refs/heads/master@{#468440}

content/test/gpu/gpu_tests/webgl2_conformance_expectations.py

@@ -719,6 +719,12 @@ class WebGL2ConformanceExpectations(WebGLConformanceExpectations):
     self.Flaky('deqp/functional/gles3/multisample.html',
         ['linux', ('nvidia', 0x104a)], bug=714207)
 
+    # Any of these tests become flaky if they follow
+    # conformance2/samplers/samplers.html and if virtualized GL
+    # contexts are being used.
+    self.Flaky('conformance2/textures/canvas_sub_rectangle/*',
+        ['linux', 'nvidia'], bug=694359)
+
     # This test is flaky both with and without ANGLE.
     self.Flaky('deqp/functional/gles3/texturespecification/' +
         'random_teximage2d_2d.html',