Started at https://build.chromium.org/p/chromium.chromiumos/builders/ChromiumOS%20x86-generic%20Compile/builds/34869.

Error log:
=== LOG STARTS ===
[990/1847] CXX obj/headless/headless_lib/headless_net_log.o
FAILED: obj/headless/headless_lib/headless_net_log.o 
/b/c/goma_client/gomacc i686-pc-linux-gnu-g++ -B/b/c/b/ChromiumOS_x86_generic_Compile/.cros_cache/chrome-sdk/tarballs/x86-generic+9428.0.0+target_toolchain/usr/x86_64-pc-linux-gnu/i686-pc-linux-gnu/binutils-bin/2.25.51-gold -MMD -MF obj/headless/headless_lib/headless_net_log.o.d -DV8_DEPRECATION_WARNINGS -DUSE_UDEV -DUSE_ASH=1 -DUSE_AURA=1 -DUSE_NSS_CERTS=1 -DUSE_OZONE=1 -DFULL_SAFE_BROWSING -DSAFE_BROWSING_CSD -DSAFE_BROWSING_DB_LOCAL -DCHROMIUM_BUILD -DFIELDTRIAL_TESTING_ENABLED -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D_FORTIFY_SOURCE=2 -DOS_CHROMEOS -DNDEBUG -DNVALGRIND -DDYNAMIC_ANNOTATIONS_ENABLED=0 -DHEADLESS_IMPLEMENTATION -DUSE_EGL -DGOOGLE_PROTOBUF_NO_RTTI -DGOOGLE_PROTOBUF_NO_STATIC_INITIALIZER -DHAVE_PTHREAD -D__CHROMEOS__ -DV8_USE_EXTERNAL_STARTUP_DATA -DU_USING_ICU_NAMESPACE=0 -DU_ENABLE_DYLOAD=0 -DU_STATIC_IMPLEMENTATION -DICU_UTIL_DATA_IMPL=ICU_UTIL_DATA_FILE -DSK_IGNORE_LINEONLY_AA_CONVEX_PATH_OPTS -DSK_SUPPORT_GPU=1 -DMESA_EGL_NO_X11_HEADERS -I../.. -Igen -I../../third_party/khronos -I../../gpu -I../../third_party/libwebp -I../../third_party/protobuf/src -Igen/protoc_out -I../../third_party/protobuf/src -I../../third_party/boringssl/src/include -I/b/c/b/ChromiumOS_x86_generic_Compile/.cros_cache/chrome-sdk/tarballs/x86-generic+9428.0.0+sysroot_chromeos-base_chromeos-chrome.tar.xz/usr/include/nss -I/b/c/b/ChromiumOS_x86_generic_Compile/.cros_cache/chrome-sdk/tarballs/x86-generic+9428.0.0+sysroot_chromeos-base_chromeos-chrome.tar.xz/usr/include/nspr -I../../breakpad/src -I../../third_party/ced/src -I../../third_party/icu/source/common -I../../third_party/icu/source/i18n -I../../third_party/libwebm/source -I../../skia/config -I../../skia/ext -I../../third_party/skia/include/c -I../../third_party/skia/include/config -I../../third_party/skia/include/core -I../../third_party/skia/include/effects -I../../third_party/skia/include/images -I../../third_party/skia/include/lazy -I../../third_party/skia/include/pathops -I../../third_party/skia/include/pdf -I../../third_party/skia/include/pipe -I../../third_party/skia/include/ports -I../../third_party/skia/include/utils -I../../third_party/skia/include/gpu -I../../third_party/skia/src/gpu -I../../third_party/skia/src/sksl -Igen -I../../third_party/WebKit -Igen/third_party/WebKit -I../../v8/include -Igen/v8/include -I../../third_party/mesa/src/include -fno-strict-aliasing -Wno-builtin-macro-redefined -D__DATE__= -D__TIME__= -D__TIMESTAMP__= -funwind-tables -fPIC -pipe -B../../third_party/binutils/Linux_x64/Release/bin -m32 -msse2 -mfpmath=sse -mmmx -pthread -Wall -Werror -Wno-unused-local-typedefs -Wno-maybe-uninitialized -Wno-missing-field-initializers -Wno-unused-parameter -O2 -fno-ident -fdata-sections -ffunction-sections -fomit-frame-pointer -g2 -gsplit-dwarf --sysroot=../../../.cros_cache/chrome-sdk/tarballs/x86-generic+9428.0.0+sysroot_chromeos-base_chromeos-chrome.tar.xz -fvisibility=hidden -fvisibility-inlines-hidden -std=gnu++11 -Wno-narrowing -fno-rtti -fno-exceptions -march=i686 -pipe -march=i686 -pipe -pipe -march=i686 -mfpmath=sse -mmmx -msse -msse2 -msse3 -D__google_stl_debug_vector=1 -c ../../headless/lib/browser/headless_net_log.cc -o obj/headless/headless_lib/headless_net_log.o
../../headless/lib/browser/headless_net_log.cc: In function 'std::unique_ptr<base::Value> headless::{anonymous}::GetHeadlessConstants()':
../../headless/lib/browser/headless_net_log.cc:36:10: error: cannot bind 'std::unique_ptr<base::DictionaryValue>' lvalue to 'std::unique_ptr<base::DictionaryValue>&&'
   return constants_dict;
          ^
In file included from /b/c/b/ChromiumOS_x86_generic_Compile/.cros_cache/chrome-sdk/tarballs/x86-generic+9428.0.0+target_toolchain/usr/x86_64-pc-linux-gnu/i686-pc-linux-gnu/gcc-bin/4.9.x/../../../../lib/gcc/i686-pc-linux-gnu/4.9.x/include/g++-v4/memory:81:0,
                 from ../../headless/lib/browser/headless_net_log.h:8,
                 from ../../headless/lib/browser/headless_net_log.cc:5:
/b/c/b/ChromiumOS_x86_generic_Compile/.cros_cache/chrome-sdk/tarballs/x86-generic+9428.0.0+target_toolchain/usr/x86_64-pc-linux-gnu/i686-pc-linux-gnu/gcc-bin/4.9.x/../../../../lib/gcc/i686-pc-linux-gnu/4.9.x/include/g++-v4/bits/unique_ptr.h:220:2: note: initializing argument 1 of 'std::unique_ptr<_Tp, _Dp>::unique_ptr(std::unique_ptr<_Up, _Ep>&&) [with _Up = base::DictionaryValue; _Ep = std::default_delete<base::DictionaryValue>; <template-parameter-2-3> = void; _Tp = base::Value; _Dp = std::default_delete<base::Value>]'
  unique_ptr(unique_ptr<_Up, _Ep>&& __u) noexcept
  ^
../../headless/lib/browser/headless_net_log.cc:37:1: error: control reaches end of non-void function [-Werror=return-type]
 }
 ^
cc1plus.elf: all warnings being treated as errors
=== LOG ENDS ===

FindIt claims that https://chromium.googlesource.com/chromium/src/+/b9a42ab6f073b057c8c05d2bfc799dcc8082e2a6 is the culprit and created a rollback CL https://codereview.chromium.org/2839153002/.

I will confirm whether the CL above is the culprit.