gn: make some warning flags match gyp more closely

This fixes differences in -Wno-unused-function and
-Wno-unneeded-internal-declaration.

It also changes the diff'ing script to accept

-Wendif-labels
-Wextra
-Wsign-compare

when in GN but not gyp, because they're more conservative. These
mostly happen when GN declares a lib chromium_code, but gyp doesn't.

Also, remove some defines from the dont_care, as I'm not sure that
we don't care yet.

And some minor tweaks to diff script output.

R=brettw@chromium.org
BUG=335824

Review URL: https://codereview.chromium.org/555343002

Cr-Commit-Position: refs/heads/master@{#294229}

• breakpad/BUILD.gn[diff]
• build/config/compiler/BUILD.gn[diff]
• third_party/cython/rules.gni[diff]
• tools/gn/bin/gyp_flag_compare.py[diff]