OS: Chrome

BuildPackages failure are seen on the following informational builders:
x86-alex-tot-chrome-pfq-informational
peach_pit-tot-chrome-pfq-informational
veyron_minnie-tot-chrome-pfq-informational

Selected error message:
chromeos-chrome-55.0.2862.0_alpha-r1: ERROR at //chrome/BUILD.gn:1358:5: Undefined identifier.
chromeos-chrome-55.0.2862.0_alpha-r1:     public_deps += [ ":default_apps" ]
chromeos-chrome-55.0.2862.0_alpha-r1:     ^----------
chromeos-chrome-55.0.2862.0_alpha-r1: See //BUILD.gn:168:7: which caused the file to be included.
chromeos-chrome-55.0.2862.0_alpha-r1:       "//chrome",
chromeos-chrome-55.0.2862.0_alpha-r1:       ^---------
chromeos-chrome-55.0.2862.0_alpha-r1:  * ERROR: chromeos-base/chromeos-chrome-55.0.2862.0_alpha-r1::chromiumos failed (configure phase):
chromeos-chrome-55.0.2862.0_alpha-r1:  *   (no error message)
chromeos-chrome-55.0.2862.0_alpha-r1:  * 
chromeos-chrome-55.0.2862.0_alpha-r1:  * Call stack:
chromeos-chrome-55.0.2862.0_alpha-r1:  *     ebuild.sh, line  93:  Called src_configure
chromeos-chrome-55.0.2862.0_alpha-r1:  *   environment, line 3765:  Called die
chromeos-chrome-55.0.2862.0_alpha-r1:  * The specific snippet of code:
chromeos-chrome-55.0.2862.0_alpha-r1:  *           ${EGN} gen "${CHROME_ROOT}/src/${BUILD_OUT_SYM}/${BUILDTYPE}" --args="${GN_ARGS}" --root="${CHROME_ROOT}/src" || die;
chromeos-chrome-55.0.2862.0_alpha-r1:  * 
See more at https://uberchromegw.corp.google.com/i/chromeos.chrome/builders/peach_pit-tot-chrome-pfq-informational/builds/2897/steps/BuildPackages/logs/stdio.

The suspicious CL:  https://codereview.chromium.org/2336033002