Suppress flakey GPU test WebglConformance_conformance2_rendering_blitframebuffer_test

WebglConformance_conformance2_rendering_blitframebuffer_test fails sometimes
on Linux Nvidia Release FYI. Mark as flaky.

BUG=906889
TBR=kbr@chromium.org

Change-Id: I3ebe2f384926ca38b9b8d592cac29014d676fb0a
Reviewed-on: https://chromium-review.googlesource.com/c/1343528Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Commit-Queue: Robert Kroeger <rjkroege@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609592}

content/test/gpu/gpu_tests/webgl2_conformance_expectations.py

@@ -970,6 +970,8 @@ class WebGL2ConformanceExpectations(WebGLConformanceExpectations):
         ['linux', ('nvidia', 0x1cb3)], bug=780706)
     self.Flaky('conformance2/rendering/canvas-resizing-with-pbo-bound.html',
         ['linux', 'nvidia'], bug=906846)
+    self.Flaky('conformance2/rendering/blitframebuffer-test.html',
+        ['linux', 'nvidia'], bug=906889)
 
     # Linux NVIDIA Quadro P400, OpenGL backend
     self.Fail('conformance/limits/gl-max-texture-dimensions.html',