Allow using goma on Windows with symbol_level == 2

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 b07a50722c287960be0eac731043f3343c16ed33..787990a0a62b2fd67d80bc37548ceb5288f0ec4a 100644
--- a/build/config/compiler/compiler.gni
+++ b/build/config/compiler/compiler.gni
@@ -98,6 +98,8 @@ if (symbol_level == -1) {
     # by visual studio (repeated in every .obj file) makes linker
     # memory consumption and link times unsustainable (crbug.com/630074).
     # Clang on windows does not have this issue.
+    # If you use is_win_fastlink = true then you can set symbol_level = 2 when
+    # using goma.
     symbol_level = 1
   } else if ((!is_nacl && !is_linux) || is_debug || is_official_build ||
              is_chromecast) {
@@ -111,6 +113,12 @@ if (symbol_level == -1) {
   } else {
     symbol_level = 0
   }
+} else if (symbol_level == 2) {
+  if (is_win) {
+    # See crbug.com/630074
+    assert(is_win_fastlink || !use_goma,
+           "Goma builds that use symbol_level 2 must use is_win_fastlink.")
+  }
 }
 
 # Assert that the configuration isn't going to hit https://crbug.com/648948.