Add fake Chromium DEPS entry

Adds a fake Chromium DEPS entry which will never actually be pulled in.
This is the same workaround that ANGLE uses to get Skia autorollers to
work with the Chromium -> ANGLE DEPS roller since autorollers need to be
set up to watch a particular dependency.

Bug: 452840620
Change-Id: If7430ebe4c281cd87a7e1520b64ac18486317698
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/272134
Commit-Queue: Brian Sheedy <bsheedy@google.com>
Auto-Submit: Brian Sheedy <bsheedy@google.com>
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>

DEPS

diff --git a/DEPS b/DEPS
index 9b573aa..7f88511 100644
--- a/DEPS
+++ b/DEPS
@@ -64,9 +64,14 @@
   # Set to True by Chromium if syncing from a Chromium checkout.
   'build_with_chromium': False,
 
+  # Version of Chromium the DEPS entries synced by scripts/roll_chromium_deps.py
+  # were last synced to.
   # NOTE: This is not currently accurate since no Chromium -> Dawn roll has
   # been performed yet.
   'chromium_revision': 'c2d941cd12644d6b893c305b1904e358727d597d',
+  # We never want to actually checkout Chromium, but we need a fake DEPS entry
+  # in order for the Chromium -> Dawn DEPS autoroller to work.
+  'checkout_dummy_chromium': False,
 }
 
 deps = {
@@ -593,6 +598,15 @@
     'url': '{chromium_git}/chromium/src/base/allocator/partition_allocator.git@fae4df38cef9720a13dd55a6b1d20600919e671b',
     'condition': 'dawn_standalone',
   },
+
+  # We never want to actually checkout Chromium, but we need a fake DEPS entry
+  # in order for the Chromium -> Dawn DEPS autoroller to work. Note that this
+  # only currently works due to an explicit exception in presubmit checks
+  # https://source.chromium.org/chromium/chromium/tools/depot_tools/+/dac161882feaedaabcf99db47726a999e4834a13:presubmit_canned_checks.py;l=2139
+  'third_party/dummy_chromium': {
+    'url': '{chromium_git}/chromium/src.git' + '@' + Var('chromium_revision'),
+    'condition': 'checkout_dummy_chromium',
+  },
 }
 
 hooks = [

scripts/roll_chromium_deps.py

diff --git a/scripts/roll_chromium_deps.py b/scripts/roll_chromium_deps.py
index a00253b..2c774ec 100755
--- a/scripts/roll_chromium_deps.py
+++ b/scripts/roll_chromium_deps.py
@@ -684,6 +684,9 @@
     chromium_deps = _parse_deps_file(
         _read_remote_chromium_file('DEPS', revision_range.new_revision))
     changed_entries = _get_changed_deps_entries(dawn_deps, chromium_deps)
+    # We change the variable directly instead of using ChangedRepo since
+    # 'gclient setdep --revision' does not work for repos if there is no entry
+    # in .gitmodules.
     changed_entries.append(
         ChangedVariable(
             name=CHROMIUM_REVISION_VAR,