Suppress WebGL conformance flakes on Linux FYI Debug (NVIDIA).

  conformance/canvas/draw-webgl-to-canvas-test.html
  conformance/extensions/webgl-depth-texture.html
  conformance/rendering/polygon-offset.html

Unknown why these are flaky on this configuration.

Bug: 918995
Change-Id: I8ab3ccf3c215999527b9d2da316097f4ce5271f2
Tbr: jdarpinian@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/1395005Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: James Darpinian <jdarpinian@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619863}

content/test/gpu/gpu_tests/webgl_conformance_expectations.py

@@ -596,6 +596,14 @@ class WebGLConformanceExpectations(GpuTestExpectations):
     self.Flaky('conformance/extensions/oes-texture-half-float-with-video.html',
         ['linux', ('nvidia', 0x1cb3)], bug=913969)
 
+    # NVIDIA P400 OpenGL, Debug
+    self.Flaky('conformance/canvas/draw-webgl-to-canvas-test.html',
+        ['linux', 'debug', ('nvidia', 0x1cb3)], bug=918995)
+    self.Flaky('conformance/extensions/webgl-depth-texture.html',
+        ['linux', 'debug', ('nvidia', 0x1cb3)], bug=918995)
+    self.Flaky('conformance/rendering/polygon-offset.html',
+        ['linux', 'debug', ('nvidia', 0x1cb3)], bug=918995)
+
     # AMD
     self.Fail('conformance/glsl/misc/fragcolor-fragdata-invariant.html',
         ['linux', 'amd'], bug=844311)