clang/win: Enable fastbuild=1 by default for now.

build/config/compiler/compiler.gni

Index: build/config/compiler/compiler.gni
diff --git a/build/config/compiler/compiler.gni b/build/config/compiler/compiler.gni
index 134f96ee9a21c96e78bbf4fffe0c5862d887df68..e2428c1e6b1ebae4fa1737cbacbac505b8859fe2 100644
--- a/build/config/compiler/compiler.gni
+++ b/build/config/compiler/compiler.gni
@@ -28,6 +28,10 @@ if (symbol_level == -1) {
     # With instrumentation enabled, debug info puts libchrome.so over 4gb, which
     # causes the linker to produce an invalid ELF. http://crbug.com/574476
     symbol_level = 0
+  } else if (is_win && is_clang) {
+    # TODO(thakis): Remove this again once building with clang/win and
+    # debug info doesn't make link.exe run for hours.
+    symbol_level = 1
   } else if (!is_linux || is_debug || is_official_build || is_chromecast) {
     # Linux is slowed by having symbols as part of the target binary, whereas
     # Mac and Windows have them separate, so in Release Linux, default them off,