[cr145] Force-enable googlechrome siso config for Brave RBE builds.

Chromium recently introduced the googlechrome config flag to restrict
remote executions for non-clang compile actions (such as mojo generators)
to only Google's official REAPI setup. This was done because "non google
builds often have issue with enabling remote executions for non-clang
compiles when they use an external REAPI."

However, Brave's RBE infrastructure is fully compatible with these remote
actions and they execute correctly in our setup. By force-enabling the
googlechrome config via -config googlechrome in .sisorc, we restore
remote execution support for mojo and other non-clang build actions in
Brave builds.

Chromium changes:
https://chromium.googlesource.com/chromium/src/+/bc0bd277c9a2fcde9989fbfe981344db0648f0b7

commit bc0bd277c9a2fcde9989fbfe981344db0648f0b7
Author: Junji Watanabe <jwata@google.com>
Date:   Thu Jan 8 18:39:35 2026 -0800

    siso: Introduce googlechrome config to enable remote executions for non clang compile actions

    Non google builds often have issue with enabling remote executions
    for non-clang compiles when they use an external REAPI.
    To make the transition to Siso smooth, this CL enables remote executions
    only for `-config googlechrome` build. backend_config/google.star now
    has `googlechrome` as a default flag.

    Bug: 424659245, 472063697
    Change-Id: I3b39d395f4130b61891ab6b1f46517bde00c8c32
    Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7409934
    Reviewed-by: Fumitoshi Ukai <ukai@google.com>
    Commit-Queue: Junji Watanabe <jwata@google.com>
    Cr-Commit-Position: refs/heads/main@{#1566686}

https://chromium.googlesource.com/chromium/src/+/5c12bc8f387c732614868975dc0ee7e9b8e25f94

commit 5c12bc8f387c732614868975dc0ee7e9b8e25f94
Author: Fumitoshi Ukai <ukai@google.com>
Date:   Fri Jan 9 01:41:57 2026 -0800

    siso: enable googlechrome only for Google RBE

    external folks may use google.star without setting reapi_address
    etc in .gclient custom_vars.

    Check reapi_address flag to enable googlechrome config in google.star.

    Bug: 424659245, 472892242, 472063697
    Change-Id: Ife9337352537c7583439fd684da3924accbd918f
    Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7414509
    Auto-Submit: Fumitoshi Ukai <ukai@google.com>
    Reviewed-by: Junji Watanabe <jwata@google.com>
    Commit-Queue: Fumitoshi Ukai <ukai@google.com>
    Cr-Commit-Position: refs/heads/main@{#1566817}

third_party/reclient_configs/brave_custom/brave_custom.py

@@ -116,6 +116,10 @@ def configure_sisorc():
         ninja_flags.append('-fs_min_flush_timeout 300s')
         # Use byte stream for most files as it is compression-aware.
         ninja_flags.append('-reapi_byte_stream_read_threshold 1024')
+        # Enable googlechrome config to build most targets with RBE. This is
+        # disabled by default in Chromium to make only clang actions be
+        # RBE-buildable, but we can build most tarets in Brave RBE infra.
+        ninja_flags.append('-config googlechrome')
 
     if cache_dir := os.environ.get('SISO_CACHE_DIR'):
         # `-cache_dir` and `-local_cache_enable` to use a local disk cache for