Revert "Roll GN from 55ad154c..dfcbc6fe"

This reverts commit 32b629e.

Reason for revert:
This broke the cros builder:
https://ci.chromium.org/ui/p/chrome/builders/ci/linux-chromeos-chrome/12015/overview

Error:
ERROR at //third_party/protobuf/proto_library.gni:118:3: Unknown function.
set_sources_assignment_filter([])

This roll includes the change:
55e535a6 Remove `set_sources_assignment_filter` function

Seems like this change was already rolled back because of a similar error in chromeos/assistant:
https://bugs.chromium.org/p/chromium/issues/detail?id=1142554

But it seems there is still a problem with //third_party/protobuf

Original change's description:
> Roll GN from 55ad154c..dfcbc6fe
>
> Generated by //tools/gn/roll_gn.py
>
> https://gn.googlesource.com/gn/+log/55ad154c..dfcbc6fe
>
>     dfcbc6fe [xcode] Fix detection of XCTests by Xcode 12.0+
>     55e535a6 Remove `set_sources_assignment_filter` function
>     8fe02009 Add "else" to the list of gn keyword
>     d7cf6238 Address readme feedback.
>     ef13fe92 Change python to python3
>     09c9e5ee Fix some typos
>     335dd456 Fix build for riscv64 arch
>     94bda7cc Reword command format error messages
>     5f30bbf9 Add more README documentation.
>
> Bug: 1018739, 1174473, 1071055, 1103230
> Change-Id: I7af4db983f6a3ade9e667bf4ec9a55387655b7f1
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2681945
> Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
> Commit-Queue: Dirk Pranke <dpranke@google.com>
> Reviewed-by: Dirk Pranke <dpranke@google.com>
> Cr-Commit-Position: refs/heads/master@{#852251}

TBR=dpranke@google.com,sdefresne@chromium.org,chromium-scoped@luci-project-accounts.iam.gserviceaccount.com

Change-Id: Idf177b276051713e76cff54c2e2ffc15d6cc9900
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1018739
Bug: 1174473
Bug: 1071055
Bug: 1103230
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2685099
Reviewed-by: Roger Tawa <rogerta@chromium.org>
Reviewed-by: Kristi Park <kristipark@chromium.org>
Commit-Queue: Roger Tawa <rogerta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#852288}

DEPS

@@ -368,7 +368,7 @@ vars = {
   # revisions.
 
   # GN CIPD package version.
-  'gn_version': 'git_revision:dfcbc6fed0a8352696f92d67ccad54048ad182b3',
+  'gn_version': 'git_revision:55ad154c961d8326315b1c8147f4e504cd95e9e6',
 
   # Also, if you change these, update buildtools/DEPS too. Also update the
   # libc++ svn_revision in //buildtools/deps_revisions.gni.

buildtools/DEPS

@@ -14,7 +14,7 @@ vars = {
   #
 
   # GN CIPD package version.
-  'gn_version': 'git_revision:dfcbc6fed0a8352696f92d67ccad54048ad182b3',
+  'gn_version': 'git_revision:55ad154c961d8326315b1c8147f4e504cd95e9e6',
 
   # By default, do not checkout the re-client binaries.
   'checkout_reclient': False,