Suppress multi-context-sampler-test on macOS nvidia

conformance2/samplers/multi-context-sampler-test.html has become flaky,
reporting and error in SharedImageStub with subsequent context loss.

So far this has only been seen on the nvidia mac builds. So only suppressing
there for now while the root cause it addressed.

TBR=kbr@chromium.org

Bug: 965209
Change-Id: I2c7d0ae78b83ee84a03de648e37fe00cb3ea4b09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1637288Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664727}

content/test/gpu/gpu_tests/test_expectations/webgl2_conformance_expectations.txt

@@ -394,6 +394,7 @@ crbug.com/911772 [ mac nvidia-0xfe9 ] conformance2/textures/canvas/tex-3d-r16f-r
 crbug.com/929398 [ mac nvidia-0xfe9 ] conformance2/glsl3/tricky-loop-conditions.html [ Failure ]
 crbug.com/934556 [ mac nvidia-0xfe9 ] conformance2/textures/canvas_sub_rectangle/tex-3d-r11f_g11f_b10f-rgb-unsigned_int_10f_11f_11f_rev.html [ RetryOnFailure ]
 crbug.com/952282 [ mac nvidia-0xfe9 ] conformance2/rendering/framebuffer-unsupported.html [ RetryOnFailure ]
+crbug.com/965209 [ mac nvidia-0xfe9 ] conformance2/samplers/multi-context-sampler-test.html [ RetryOnFailure ]
 crbug.com/756537 [ mac nvidia ] deqp/functional/gles3/shaderoperator/* [ Failure ]
 crbug.com/907599 [ mac nvidia debug ] conformance/textures/video/tex-2d-rgb-rgb-unsigned_short_5_6_5.html [ RetryOnFailure ]