Suppress a WebGL2 flaky test on Linux AMD R7 240

conformance2/textures/image_bitmap_from_video/
tex-2d-rgba32f-rgba-float.html

BUG=701138
TBR=kbr@chromium.org
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/2753643002
Cr-Commit-Position: refs/heads/master@{#456841}