Clarify SQlite threadsafety compile flags.

third_party/sqlite/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("//testing/libfuzzer/fuzzer_test.gni")
 
 declare_args() {
   # Controls whether the build should uses the version of sqlite3 library
   # shipped with the system (currently only supported on iOS) or the one
   # shipped with Chromium source.
@@ skipping 45 matching lines @@
       "SQLITE_DISABLE_FTS4_DEFERRED",
       "SQLITE_ENABLE_ICU",
       "SQLITE_ENABLE_MEMORY_MANAGEMENT",
       "SQLITE_SECURE_DELETE",
 
       # Custom flag to tweak pcache pools.
       # TODO(shess): This shouldn't use faux-SQLite naming.
       "SQLITE_SEPARATE_CACHE_POOLS",
 
       # TODO(shess): SQLite adds mutexes to protect structures which cross
-      # threads.  In theory Chromium should be able to turn this off for a
-      # slight speed boost.
-      "THREADSAFE",
+      # threads. In theory Chromium should be able to turn this to "2" which
+      # should give a slight speed boost. "2" is safe as long as a single
+      # connection is not used by more than one thread at a time.
+      "SQLITE_THREADSAFE=1",
 
       # SQLite can spawn threads to sort in parallel if configured
       # appropriately.  Chromium doesn't configure SQLite for that, and would
       # prefer to control distribution to worker threads.
       "SQLITE_MAX_WORKER_THREADS=0",
 
       # Allow 256MB mmap footprint per connection.  Should not be too open-ended
       # as that could cause memory fragmentation.  50MB encompasses the 99th
       # percentile of Chrome databases in the wild.
       # TODO(shess): A 64-bit-specific value could be 1G or more.
@@ skipping 211 matching lines @@
 fuzzer_test("sqlite3_ossfuzz_fuzzer") {
   # TODO(mmoroz, shess): remove fuzz/ossfuzz.c after next sqlite3 update.
   sources = [
     "fuzz/ossfuzz.c",
   ]
   deps = [
     ":sqlite",
   ]
   dict = "fuzz/sql.dict"
 }