Turn on use_prebuilt_instrumented_libraries by default if is_msan is set.

We only use prebuilt instrumented libraries with msan, so use the right
default for that, and asser that it's not used in any other build configs.

Bug: 1002971
Change-Id: I23c8099394af711a9e28027f9c3093f9f6fd18b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1797468
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Evgenii Stepanov <eugenis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695889}

build/config/sanitizers/sanitizers.gni

@@ -47,11 +47,6 @@ declare_args() {
   # chain of stores leading from allocation site to use site.
   msan_track_origins = 2
 
-  # Use dynamic libraries instrumented by one of the sanitizers instead of the
-  # standard system libraries. Set this flag to download prebuilt binaries from
-  # GCS.
-  use_prebuilt_instrumented_libraries = false
-
   # Use dynamic libraries instrumented by one of the sanitizers instead of the
   # standard system libraries. Set this flag to build the libraries from source.
   use_locally_built_instrumented_libraries = false
@@ -122,6 +117,15 @@ declare_args() {
   archive_seed_corpus = true
 }
 
+declare_args() {
+  # Use dynamic libraries instrumented by one of the sanitizers instead of the
+  # standard system libraries. Set this flag to download prebuilt binaries from
+  # GCS.
+  # TODO(thakis): Stop setting this in bot configs, then stop making this
+  # an arg. It should be set exactly when is_msan is on.
+  use_prebuilt_instrumented_libraries = is_msan
+}
+
 # Disable sanitizers for non-default toolchains.
 if (current_toolchain != default_toolchain) {
   is_asan = false
@@ -147,6 +151,9 @@ if (current_toolchain != default_toolchain) {
   use_sanitizer_coverage = false
 }
 
+assert(is_msan == use_prebuilt_instrumented_libraries,
+       "msan requires using instrumented libraries, but nothing else does")
+
 # Whether we are doing a fuzzer build. Normally this should be checked instead
 # of checking "use_libfuzzer || use_afl" because often developers forget to
 # check for "use_afl".
@@ -194,9 +201,6 @@ if (use_libfuzzer && is_linux) {
     # relies on LEAK_SANITIZER define to avoid false positives.
     is_lsan = true
   }
-  if (is_msan) {
-    use_prebuilt_instrumented_libraries = true
-  }
 }
 
 # MSan only links Chrome properly in release builds (brettw -- 9/1/2015). The