Enable ThinLTO for POSIX LTO by default on Linux, take 2.

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 0bf51f949defa4acaf199548e94807d30b785d8d..26377e5edf8627a9f14a622c3b4c617b5ed39007 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -493,6 +493,11 @@ config("compiler") {
           "-Wl,--thinlto-jobs=8",
           "-Wl,--thinlto-cache-dir=" +
               rebase_path("$root_out_dir/thinlto-cache", root_build_dir),
+
+          # Limit the size of the ThinLTO cache to 10% of available disk space.
+          # TODO(pcc): Change the limit from a percentage to an absolute size
+          # (10-20GB) once that feature lands in LLVM.
+          "-Wl,--thinlto-cache-policy,cache_size=10%",
         ]
       } else {
         ldflags += [ "-Wl,-plugin-opt,jobs=8" ]