Full output from recompilation:

[1/73] ACTION //headless:gen_devtools_client_api(//build/toolchain/linux:clang_x64)
[2/73] STAMP obj/headless/gen_devtools_client_api.stamp
[3/73] STAMP obj/headless/headless_lib.inputdeps.stamp
[4/73] STAMP obj/headless/headless_shell.inputdeps.stamp
[5/73] STAMP obj/headless/headless_shell_lib.inputdeps.stamp
[6/73] CXX obj/headless/headless_lib/types_console.o
[7/73] CXX obj/headless/headless_lib/headless_devtools_manager_delegate.o
[8/73] CXX obj/headless/headless_lib/headless_web_contents_impl.o
[9/73] CXX obj/headless/headless_shell_lib/headless_shell.o
[10/73] CXX obj/headless/headless_lib/device_orientation.o
[11/73] CXX obj/headless/headless_lib/rendering.o
[12/73] CXX obj/headless/headless_lib/indexeddb.o
[13/73] CXX obj/headless/headless_lib/cache_storage.o
[14/73] CXX obj/headless/headless_lib/accessibility.o
[15/73] CXX obj/headless/headless_lib/types_animation.o
[16/73] CXX obj/headless/headless_lib/types_profiler.o
[17/73] CXX obj/headless/headless_lib/tracing.o
[18/73] CXX obj/headless/headless_lib/types_inspector.o
[19/73] CXX obj/headless/headless_lib/memory.o
[20/73] CXX obj/headless/headless_lib/application_cache.o
[21/73] CXX obj/headless/headless_lib/console.o
[22/73] CXX obj/headless/headless_lib/types_dom_storage.o
[23/73] CXX obj/headless/headless_lib/types_indexeddb.o
[24/73] CXX obj/headless/headless_lib/types_database.o
[25/73] CXX obj/headless/headless_lib/css.o
[26/73] CXX obj/headless/headless_lib/runtime.o
[27/73] CXX obj/headless/headless_lib/headless_devtools_client_impl.o
[28/73] CXX obj/headless/headless_lib/types_css.o
[29/73] CXX obj/headless/headless_lib/log.o
[30/73] CXX obj/headless/headless_lib/types_dom_debugger.o
[31/73] CXX obj/headless/headless_lib/types_emulation.o
[32/73] CXX obj/headless/headless_lib/target.o
[33/73] CXX obj/headless/headless_lib/heap_profiler.o
[34/73] CXX obj/headless/headless_lib/types_layer_tree.o
[35/73] CXX obj/headless/headless_lib/types_log.o
[36/73] CXX obj/headless/headless_lib/types_heap_profiler.o
[37/73] CXX obj/headless/headless_lib/dom_storage.o
[38/73] CXX obj/headless/headless_lib/emulation.o
[39/73] CXX obj/headless/headless_lib/types_service_worker.o
[40/73] CXX obj/headless/headless_lib/network.o
[41/73] CXX obj/headless/headless_lib/io.o
[42/73] CXX obj/headless/headless_lib/database.o
[43/73] CXX obj/headless/headless_lib/types_device_orientation.o
[44/73] CXX obj/headless/headless_lib/profiler.o
[45/73] CXX obj/headless/headless_lib/layer_tree.o
[46/73] CXX obj/headless/headless_lib/input.o
[47/73] CXX obj/headless/headless_lib/animation.o
[48/73] CXX obj/headless/headless_lib/dom_tree_extractor.o
[49/73] CXX obj/headless/headless_lib/dom_debugger.o
[50/73] CXX obj/headless/headless_lib/types_accessibility.o
[51/73] CXX obj/headless/headless_lib/types_runtime.o
[52/73] CXX obj/headless/headless_lib/types_memory.o
[53/73] CXX obj/headless/headless_lib/inspector.o
[54/73] CXX obj/headless/headless_lib/types_rendering.o
[55/73] CXX obj/headless/headless_lib/debugger.o
[56/73] CXX obj/headless/headless_lib/security.o
[57/73] CXX obj/headless/headless_lib/types_application_cache.o
[58/73] CXX obj/headless/headless_lib/types_cache_storage.o
[59/73] CXX obj/headless/headless_lib/types_security.o
[60/73] CXX obj/headless/headless_lib/service_worker.o
[61/73] CXX obj/headless/headless_lib/types_io.o
[62/73] CXX obj/headless/headless_lib/types_input.o
[63/73] CXX obj/headless/headless_lib/types_tracing.o
[64/73] CXX obj/headless/headless_lib/page.o
[65/73] CXX obj/headless/headless_lib/types_debugger.o
[66/73] CXX obj/headless/headless_lib/types_network.o
[67/73] CXX obj/headless/headless_lib/types_page.o
[68/73] CXX obj/headless/headless_lib/dom.o
[69/73] CXX obj/headless/headless_lib/types_target.o
[70/73] CXX obj/headless/headless_lib/types_dom.o
[71/73] AR obj/headless/libheadless_lib.a
[72/73] AR obj/headless/libheadless_shell_lib.a
[73/73] LINK ./headless_shell

Right, looks like headless:gen_devtools_client_api is always rebuilt.

Found le bug.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fa434b649b61214ed73ed6a7b4354984ea81a2c2

commit fa434b649b61214ed73ed6a7b4354984ea81a2c2
Author: skyostil <skyostil@chromium.org>
Date: Mon Nov 28 18:28:21 2016

headless: Fix inconsistent header naming

Because of a missing 's' the generated client API was always considered out of
date.

BUG= 669077 

Review-Url: https://codereview.chromium.org/2531363002
Cr-Commit-Position: refs/heads/master@{#434699}

[modify] https://crrev.com/fa434b649b61214ed73ed6a7b4354984ea81a2c2/headless/BUILD.gn
[modify] https://crrev.com/fa434b649b61214ed73ed6a7b4354984ea81a2c2/headless/lib/browser/devtools_api/client_api_generator.py
[rename] https://crrev.com/fa434b649b61214ed73ed6a7b4354984ea81a2c2/headless/lib/browser/devtools_api/domain_types_forward_declarations_h.template
[modify] https://crrev.com/fa434b649b61214ed73ed6a7b4354984ea81a2c2/headless/lib/browser/devtools_api/domain_types_h.template