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

Review URL: https://codereview.chromium.org/1095203007

Cr-Commit-Position: refs/heads/master@{#326866}

gpu/gpu.gyp

@@ -750,6 +750,15 @@
           'sources': [
             'gpu_unittests.isolate',
           ],
+          'conditions': [
+            ['use_x11==1',
+              {
+                'dependencies': [
+                  '../tools/xdisplaycheck/xdisplaycheck.gyp:xdisplaycheck',
+                ],
+              }
+            ],
+          ],
         },
       ],
     }],