is_lsan=true requires is_asan=true -- add a gn-time assert to check for that.

Bug: none
Change-Id: Iacc611282edb2d53bf213b75c07e4b724b2ac0ac
Reviewed-on: https://chromium-review.googlesource.com/989246Reviewed-by: Chris Mumford <cmumford@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547436}

build/config/sanitizers/sanitizers.gni

@@ -201,6 +201,8 @@ assert(!is_debug || !(is_msan || is_ubsan || is_ubsan_null || is_ubsan_vptr),
 assert(!is_msan || (is_linux && current_cpu == "x64"),
        "MSan currently only works on 64-bit Linux and ChromeOS builds.")
 
+assert(!is_lsan || is_asan, "is_lsan = true requires is_asan = true also.")
+
 # ASAN build on Windows is not working in debug mode. Intercepting memory
 # allocation functions is hard on Windows and not yet implemented in LLVM.
 assert(!is_win || !is_debug || !is_asan,