We need to define use_x11 variable to build vulkan_tests in Linux

Here is the error message:

[1/1] Regenerating ninja files
FAILED: build.ninja
/home/joone/git/chromium/src/buildtools/linux64/gn --root=/home/joone/git/chromium/src -q gen .
ERROR at //gpu/vulkan/BUILD.gn:20:14: Undefined identifier
      assert(use_x11, "Vulkan only support x11 at this point.")
                 ^------
See //cc/BUILD.gn:486:5: which caused the file to be included.
    "//gpu/vulkan:features",
        ^----------------------
ninja: error: rebuilding 'build.ninja': subcommand failed

BUG=none
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/2759353003
Cr-Commit-Position: refs/heads/master@{#458605}