Enable use_dummy_lastchange=true for linux_chromium_rel_ng builder

This is to take stats of cache hit rate on swarming test execution.

I will record stats to the issue few days later.

I changed dummy value due to some test failure.

Bug: 869348
Change-Id: Ied830d0e783065e3aad48f066ad6b471b735fe00
Reviewed-on: https://chromium-review.googlesource.com/1157737Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579787}

build/util/LASTCHANGE.dummy

-LASTCHANGE=thisisdummylastchange-thisisdummylastchange
+LASTCHANGE=0000000000000000000000000000000000000000-0000000000000000000000000000000000000000

tools/mb/mb_config.pyl

@@ -605,7 +605,7 @@
       # on the continuous builder.
       'linux_chromium_msan_rel_ng': 'msan_release_bot',
 
-      'linux_chromium_rel_ng': 'gpu_tests_release_trybot_no_symbols',
+      'linux_chromium_rel_ng': 'gpu_tests_release_trybot_no_symbols_use_dummy_lastchange',
       'linux_chromium_tsan_rel_ng': 'tsan_disable_nacl_release_trybot',
       'linux_chromium_ubsan_rel_ng': 'ubsan_vptr_release_trybot',
       'linux_layout_tests_layout_ng': 'release_trybot',
@@ -1328,6 +1328,10 @@
       'gpu_tests', 'release_trybot', 'no_symbols',
     ],
 
+    'gpu_tests_release_trybot_no_symbols_use_dummy_lastchange': [
+      'gpu_tests', 'release_trybot', 'no_symbols', 'use_dummy_lastchange',
+    ],
+
     'gpu_tests_release_trybot_cm': [
       'gpu_tests', 'release_trybot', 'v8_concurrent_marking',
     ],
@@ -2058,6 +2062,12 @@
       'gn_args': 'use_lld=false',
     },
 
+    # TODO(crbug.com/869348): Remove this if we set this by default or don't use
+    # this feature.
+    'use_dummy_lastchange': {
+      'gn_args': 'use_dummy_lastchange=true',
+    },
+
     'use_vaapi': {
       'gn_args': 'use_vaapi=true',
     },