This helps clang-format a bit (without these, it removes the empty
line before the namespace's closing '}'), and since we want these
comments anyway, just add them.

Patch created by running (in src/out/Release):
time ~/src/llvm-rw/tools/clang/tools/extra/clang-tidy/tool/run-clang-tidy.py \
    -checks=-*,google-readability-namespace-comments -p ../.. \
    -clang-tidy-binary ~/src/llvm-build/bin/clang-tidy \
    -header-filter third_party/WebKit/Source -j8 -fix third_party/WebKit/Source

Previously I created compile_commands.json by running (in src/):
    ninja -C out/Release/ -t compdb objcxx cxx > compile_commands.json

To let clang-tidy and clang agree on which revision should be used for precompiled
header files (I did this on Mac), I gypped like so and built 'blink' and 'webcore' normally
before running clang-tidy:

GYP_DEFINES='make_clang_dir=/Users/thakis/src/llvm-build clang_use_chrome_plugins=0 fastbuild=1 component=shared_library' build/gyp_chromium

This is like https://codereview.chromium.org/1629693002/, but for all of blink.
No behavior change.

BUG=580787, 403854
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/1627713002 .

Cr-Commit-Position: refs/heads/master@{#371243}