GN: Fix flags writing for targets with unsupported PCHs

This fixes an issue that led to empty compiler flags being written
in the unusual case of a target with a PCH in conjunction with a toolchain
that doesn't support PCHs.

BUG=649899

Review-Url: https://codereview.chromium.org/2410253002
Cr-Original-Commit-Position: refs/heads/master@{#424669}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 52de7c17a1f9e4b3a2ed57d41afd131c0f4f8e69
1 file changed
tree: 18439be5649b3ff32c5436754d0ddc8f313b77ba
  1. tools/