Allow using goma on Windows with symbol_level == 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 9d651ad27157f85f6aa0d723bf677abf284e614a..9202fd0abff3024778867f35b607ea9543e71864 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -1580,7 +1580,12 @@ config("afdo") {
 # Full symbols.
 config("symbols") {
   if (is_win) {
-    cflags = [ "/Zi" ]  # Produce PDB file, no edit and continue.
+    if (use_goma) {
+      # Note that this requires is_win_fastlink, enforced elsewhere.
+      cflags = [ "/Z7" ]  # Debug information in the .obj files.

[scottmg, 2017/02/01 21:56:43]

Can you not just set is_win_fastlink here instead of asserting in the other
location? (That's really a question, I don't know if it'll work or not.)

[grt (UTC plus 2), 2017/02/01 22:06:37]

1) You are awesome, Bruce. I was just searching for goma advice and came across
this CL.
2) I like the idea of flipping one switch to get sane behavior rather than
instructions like "set A and B to get cool feature". I believe that other GN
knobs work this way, so I hope it's possible here.

+    } else {
+      cflags = [ "/Zi" ]  # Produce PDB file, no edit and continue.
+    }
 
     if (is_win_fastlink) {
       # Tell VS 2015+ to create a PDB that references debug