gpu: Remove testonly header files from service_sources target.

These headers are intended to be consumed by tests only, not by
production code and thus should not be listed in service_sources target.

This makes the following errors disappear:

ERROR at //gpu/command_buffer/service/gpu_scheduler_mock.h:9:11: Include
not allowed.
          ^----------------------------------
It is not in any dependency of
//gpu/command_buffer/service:service_sources
The include file is in the target(s):
//testing/gmock:gmock
which should somehow be reachable.

ERROR at //gpu/command_buffer/service/mocks.h:22:11: Include not allowed.
          ^----------------------------------
It is not in any dependency of
//gpu/command_buffer/service:service_sources
The include file is in the target(s):
//testing/gmock:gmock
which should somehow be reachable.

Tested on Linux with the following command lines:

$ gn gen out-gn/Debug
$ gn check out-gn/Debug //gpu/command_buffer/service/*

BUG=367595,376000
TEST=see above
R=sievers@chromium.org

Review URL: https://codereview.chromium.org/1114633003

Cr-Commit-Position: refs/heads/master@{#327384}

gpu/command_buffer/service/BUILD.gn

@@ -76,7 +76,6 @@ source_set("service_sources") {
     "gles2_cmd_validation_implementation_autogen.h",
     "gpu_scheduler.cc",
     "gpu_scheduler.h",
-    "gpu_scheduler_mock.h",
     "gpu_state_tracer.cc",
     "gpu_state_tracer.h",
     "gpu_switches.cc",
@@ -100,7 +99,6 @@ source_set("service_sources") {
     "mailbox_manager_sync.h",
     "memory_program_cache.cc",
     "memory_program_cache.h",
-    "mocks.h",
     "program_cache.cc",
     "program_cache.h",
     "program_manager.cc",