Verify gn_all on iOS (i.e., that GN builds everything GYP builds)

We need to go through the GYP and GN builds and verify that we're actually building everything we should be.

In particular, we need to address

https://cs.chromium.org/chromium/src/BUILD.gn?rcl=0&l=482

and see what's still missing.

From:

https://bugs.chromium.org/p/chromium/issues/detail?id=431177#c74 by thakis@:

I did a build of all targets with both gn and gyp. These test binaries are missing in a gn build:

$ comm -2 -3 <(cd out/Release && ls *tests) <(cd out/gn && ls *tests)
...

https://bugs.chromium.org/p/chromium/issues/detail?id=431177#c74 by thakis@:

I then compared if the tests in the binaries built in both builds are identical like so:

comm -1 -2 <(cd out/Release && ls *tests) <(cd out/gn && ls *tests) > tests.txt
for t in $(cat tests.txt); do echo $t; diff <(out/gn/$t --gtest_list_tests | grep -v GetParam | sort) <(out/Release/$t --gtest_list_tests | grep -v GetParam | sort); done