Disable compilation symbols on goma builds

Goma builds necessarily don't use PDBs. That means that symbols have to be put in .obj files. This means that symbols get stored redundantly, which makes the job of the linker much harder - greatly increased linker working sets and greatly increased link times.

gyp deals with this by disabling symbol generation in the compile stage for goma builds. gn has not been doing this, which has made gn goma builds *significantly* slower, unless symbol_level is explicitly set to 1.

gyp has an override switch (win_z7) but it appears that nobody uses it so for now I am leaving it unimplemented in gn.

BUG=630074

Review-Url: https://codereview.chromium.org/2174873002
Cr-Commit-Position: refs/heads/master@{#407251}

build/config/compiler/BUILD.gn

@@ -1487,7 +1487,10 @@ config("symbols") {
   if (is_win) {
     import("//build/toolchain/goma.gni")
     if (use_goma) {
-      cflags = [ "/Z7" ]  # No PDB file
+      # 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.
     }