gpu: Add missing dependency on xdisplaycheck.

gpu: Add missing dependency on xdisplaycheck.

It should fix the following error:
FAILED: cd ../../gpu; python ../tools/isolate_driver.py check --isolated
"../out/Release/gpu_unittests.isolated" --isolate
"gpu_unittests.isolate" --path-variable DEPTH .. --path-variable
PRODUCT_DIR "../out/Release " --extra-variable
"version_full=44.0.2383.0" --config-variable
"CONFIGURATION_NAME=Release" --config-variable "OS=linux"
--config-variable "asan=0" --config-variable "branding=Chromium"
--config-variable "chromeos=0" --config-variable
"component=static_library" --config-variable "disable_nacl=0"
--config-variable "enable_pepper_cdms=1" --config-variable
"enable_plugins=1" --config-variable "fastbuild=1" --config-variable
"icu_use_data_file_flag=1" --config-variable
"internal_gles2_conform_tests=0" --config-variable
"libpeer_target_type=static_library" --config-variable "lsan=0"
--config-variable "msan=0" --config-variable "target_arch=x64"
--config-variable "tsan=0" --config-variable "use_custom_libcxx=0"
--config-variable "use_instrumented_libraries=0" --config-variable
"use_prebuilt_instrumented_libraries=0" --config-variable
"use_openssl=0" --config-variable "use_ozone=0" --config-variable
"use_x11=1" --config-variable "v8_use_external_startup_data=1"
Failed to find an input file: Input file
/b/build/slave/linux_clobber/build/src/out/Release/xdisplaycheck doesn't
exist

http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_clobber_rel_ng/builds/19419/steps/compile%20%28with%20patch%29/logs/stdio

BUG=481110
TEST=trybots
R=sievers@chromium.org

Committed: https://crrev.com/f0a509791e15f92b6225c99946694fb5ca203a29
Cr-Commit-Position: refs/heads/master@{#326866}

[no sievers, 2015-04-24 20:14:53]

On 2015/04/24 18:40:12, tfarina wrote:

lgtm

[tfarina, 2015-04-24 20:21:39]

The CQ bit was checked by tfarina@chromium.org

[commit-bot: I haz the power, 2015-04-24 20:22:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1095203007/1

[commit-bot: I haz the power, 2015-04-24 20:25:57]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-24 20:27:02]

Patchset 1 (id:??) landed as
https://crrev.com/f0a509791e15f92b6225c99946694fb5ca203a29
Cr-Commit-Position: refs/heads/master@{#326866}