[bazel] Add optional deps for prebuilt bazel for bots

This will download a prebuilt bazel on Ubuntu bots if gclient variable
download_prebuilt_bazel is set. The currently pinned version is:
5.0.0-pre.20210604.6.

This is planned to only run on specific bots that use a new recipe
for building with bazel:
https://crrev.com/c/2972749

Bug: v8:11912
Change-Id: I8ff405fc03bfede4762bf4e8577cb1feab56f7c4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2972918
Auto-Submit: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Liviu Rau <liviurau@chromium.org>
Reviewed-by: Liviu Rau <liviurau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75288}

DEPS

@@ -41,6 +41,7 @@ vars = {
   'android_url': 'https://android.googlesource.com',
   'download_gcmole': False,
   'download_jsfunfuzz': False,
+  'download_prebuilt_bazel': False,
   'check_v8_header_includes': False,
 
   # GN CIPD package version.
@@ -342,6 +343,17 @@ hooks = [
         'tools/get_landmines.py',
     ],
   },
+  {
+    'name': 'bazel',
+    'pattern': '.',
+    'condition': 'download_prebuilt_bazel',
+    'action': [ 'download_from_google_storage',
+                '--bucket', 'chromium-v8-prebuilt-bazel/linux',
+                '--no_resume',
+                '-s', 'tools/bazel/bazel.sha1',
+                '--platform=linux*',
+    ],
+  },
   # Pull clang-format binaries using checked-in hashes.
   {
     'name': 'clang_format_win',

tools/bazel/.gitignore

@@ -0,0 +1 @@
+bazel

tools/bazel/bazel.sha1

@@ -0,0 +1 @@
+a85565a04bf357b2cb4bf04a1c68b83660a47ea2