Clang ToT: drop clang_use_chrome_plugins=false

I think at some point the ToT bots didn't actually build the plugins, so we had
to disable them in the build. But these days the plugins are available, and we
want the test coverage.

Bug: 960153
Change-Id: Id23c0af65ed7dce15cbb2e07fcb9ae91298644f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1599453
Commit-Queue: Hans Wennborg <hans@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Hans Wennborg <hans@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657658}

tools/mb/mb_config.pyl

@@ -2020,7 +2020,7 @@
     },
 
     'clang_tot': {
-      'gn_args': 'llvm_force_head_revision=true clang_use_chrome_plugins=false',
+      'gn_args': 'llvm_force_head_revision=true',
       'mixins': ['clang'],
     },