GN Windows build fixes

Most importantly, this passes cflags_cc to the C++ compiler.

This fixes some flag differences with the GYP build, especially when running on 64 bit.

It disables a TLS test on static debug x64 builds which also fails in GYP. This configuration is not run on the bots but is the standard GN Windows build.

BUG=251251
CQ_EXTRA_TRYBOTS=tryserver.chromium.linux:android_chromium_gn_compile_dbg,android_chromium_gn_compile_rel;tryserver.chromium.win:win8_chromium_gn_rel,win8_chromium_gn_dbg;tryserver.chromium.mac:mac_chromium_gn_rel,mac_chromium_gn_dbg

Review URL: https://codereview.chromium.org/1062743005

Cr-Commit-Position: refs/heads/master@{#326856}

diff --git a/BUILD.gn b/BUILD.gn
index 4741461..5b64b2f 100644
--- a/BUILD.gn
+++ b/BUILD.gn

@@ -763,7 +763,7 @@
     testonly = true
     deps = [
       "//ash:ash_unittests",  # FAILS 4/20/2015
-      "//base:base_unittests",  # PASSES 4/17/2015 in 32-bit, some 64-bit failures.
+      "//base:base_unittests",  # PASSES 4/20/2015
       "//cc:cc_unittests",  # PASSES 4/17/2015
       "//chrome_elf:chrome_elf_unittests",  # FAILS 4/20/2015
       "//chrome/test:browser_tests",