Skip getextension-while-pbo-bound-stability on Linux.

There are known bugs affecting this test and it's failing
intermittently on linux_optional_gpu_tests_rel.

BUG=644572
CQ_INCLUDE_TRYBOTS=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;master.tryserver.chromium.android:android_optional_gpu_tests_rel
TBR=zmo@chromium.org
NOTRY=true

Review-Url: https://codereview.chromium.org/2320033002
Cr-Commit-Position: refs/heads/master@{#417188}

content/test/gpu/gpu_tests/webgl2_conformance_expectations.py

@@ -429,6 +429,9 @@ class WebGL2ConformanceExpectations(WebGLConformanceExpectations):
         ['mac', 'intel'], bug=483282)
 
     # Linux only.
+
+    self.Skip('conformance2/misc/getextension-while-pbo-bound-stability.html',
+              ['linux'], bug=644572)
     self.Flaky('conformance/textures/video/' +
                'tex-2d-rgba-rgba-unsigned_byte.html',
                ['linux'], bug=627525)
@@ -476,10 +479,6 @@ class WebGL2ConformanceExpectations(WebGLConformanceExpectations):
     self.Fail('conformance2/reading/read-pixels-pack-parameters.html',
         ['linux', 'opengl'], bug=483282)
 
-    # Linux NVIDIA only.
-    self.Fail('conformance2/misc/getextension-while-pbo-bound-stability.html',
-        ['linux', 'nvidia'], bug=641643)
-
     # Linux NVIDIA with ANGLE only
     self.Fail('conformance/attribs/gl-disabled-vertex-attrib.html',
               ['linux', 'nvidia', 'opengl'], bug=483282)