Rework default GN symbol handling for win goma builds.

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 588db2bca8e6df2ffbedbfc18d3531a1d1d46db4..83126d04fcd26db3932ee848acb8dd3051f19bc9 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -1478,14 +1478,10 @@ config("default_optimization") {
 config("symbols") {
   if (is_win) {
     import("//build/toolchain/goma.gni")
-    if (use_goma) {
-      # Disable symbols during goma compilation because otherwise the redundant
-      # debug information (repeated in every .obj file) makes linker memory
-      # consumption and link times unsustainable (crbug.com/630074).
-      cflags = []
-    } else {
-      cflags = [ "/Zi" ]  # Produce PDB file, no edit and continue.
-    }
+    assert(!use_goma,
+           "Cannot generate full symbols w/ goma, use symbol_level=1")

[scottmg, 2016/08/03 23:37:24]

Oh, we don't have win_z7 in GN? I guess goma-win is mostly a bot thing by this
point?

+    cflags = [ "/Zi" ]  # Produce PDB file, no edit and continue.
+
     if (is_win_fastlink && visual_studio_version != "2013") {
       # Tell VS 2015+ to create a PDB that references debug
       # information in .obj and .lib files instead of copying