Add MB configs for ClangToTLinux* bots on FYI

build/config/compiler/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/config.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/toolchain.gni")
@@ skipping 72 matching lines @@
 
   # If true, optimize for size. Does not affect windows builds.
   # Linux & Mac favor speed over size.
   # TODO(brettw) it's weird that Mac and desktop Linux are different. We should
   # explore favoring size over speed in this case as well.
   optimize_for_size = is_android || is_ios
 
   # Gold icf level, values are "none", "safe", "all". If value isn't overridden,
   # default initialization is below.
   gold_icf_level = ""
+
+  # If this is set to true, or if LLVM_FORCE_HEAD_REVISION is set to 1
+  # in the environment, we use the revision in the llvm repo to determine
+  # the CLANG_REVISION to use, instead of the version hard-coded into
+  # //tools/clang/scripts/update.py. This should only be used in
+  # conjunction with setting LLVM_FORCE_HEAD_REVISION in the
+  # environment when `gclient runhooks` is run as well.
+  llvm_force_head_revision = false
+}
+
+if (is_clang && !is_nacl) {
+  # This is here so that all files get recompiled after a clang roll and
+  # when turning clang on or off. (defines are passed via the command line,
+  # and build system rebuild things when their commandline changes). Nothing
+  # should ever read this define.
+  update_args = [ "--print-revision" ]
+  if (llvm_force_head_revision) {
+    update_args += [ "--llvm-force-head-revision" ]
+  }
+  clang_revision =
+      exec_script("//tools/clang/scripts/update.py", update_args, "trim string")
 }
 
 # Apply the default logic for these values if they were not set explicitly.
 if (gold_path == false) {
   if (use_gold) {
     gold_path = rebase_path("//third_party/binutils/Linux_x64/Release/bin",
                             root_build_dir)
   } else {
     gold_path = ""
   }
@@ skipping 112 matching lines @@
     # in debug builds. Technically, these can never be null in
     # well-defined C/C++ and Clang can optimize such checks away in
     # release builds, but they may be used in asserts in debug builds.
     cflags_cc += [
       "-Wno-undefined-bool-conversion",
       "-Wno-tautological-undefined-compare",
     ]
   }
 
   if (is_clang && !is_nacl) {
-    # This is here so that all files get recompiled after a clang roll and
-    # when turning clang on or off. (defines are passed via the command line,
-    # and build system rebuild things when their commandline changes). Nothing
-    # should ever read this define.
-    defines +=
-        [ "CR_CLANG_REVISION=" + exec_script("//tools/clang/scripts/update.py",
-                                             [ "--print-revision" ],
-                                             "trim string") ]
+    defines += [ "CR_CLANG_REVISION=$clang_revision" ]

[Nico, 2016/04/24 01:52:48]

restore this to what it was (except with the added flag, optionally) and

[Dirk Pranke, 2016/04/24 20:49:18]

I'm not sure I understand what you're asking here, but perhaps you want the
comment either copied or moved from #104-107 to here? I'll go ahead and do that.

   }
 
   # Non-Mac Posix compiler flags setup.
   # -----------------------------------
   if (is_posix && !(is_mac || is_ios)) {
     if (enable_profiling && !is_debug) {
       cflags += [
         "-fno-omit-frame-pointer",
         "-g",
       ]
@@ skipping 724 matching lines @@
     # NaCl's Clang compiler and Chrome's hermetic Clang compiler will almost
     # always have different versions. Certain flags may not be recognized by
     # one version or the other.
     if (!is_nacl) {
       # Flags NaCl (Clang 3.7) does not recognize.
       cflags += [
         # TODO(thakis): Enable this, crbug.com/507717
         "-Wno-shift-negative-value",
       ]
 
-      if (exec_script("//tools/clang/scripts/update.py",
-                      [ "--print-revision" ],
-                      "trim string") != "266460-1") {
+      if (clang_revision != "266460-1") {

[Nico, 2016/04/24 01:52:48]

make this say

  if (llvm_force_head_revision)

since that's what it effectively checks

[Dirk Pranke, 2016/04/24 20:49:18]

Okay, it wasn't clear to me if the intent was "ToT" or "not this revision".

         cflags += [
           # TODO(thakis): https://crbug.com/604888
           "-Wno-undefined-var-template",
         ]
       }
     }
   }
 }
 
 # chromium_code ---------------------------------------------------------------
@@ skipping 437 matching lines @@
 
 if (is_ios || is_mac) {
   # On Mac and iOS, this enables support for ARC (automatic ref-counting).
   # See http://clang.llvm.org/docs/AutomaticReferenceCounting.html.
   config("enable_arc") {
     common_flags = [ "-fobjc-arc" ]
     cflags_objc = common_flags
     cflags_objcc = common_flags
   }
 }