Update clang-format build instructions for Mac

Requires the same LLVM_ENABLE_PROJECTS arg specified. I also removed the
duplication for the Ninja build instructions.

R=hans@chromium.org

Bug: 993206
Change-Id: I96d278120be62813a62b8c1f930b5ff55e2f34cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2082924
Auto-Submit: Tim van der Lippe <tvanderlippe@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746339}

docs/updating_clang_format_binaries.md

@@ -63,9 +63,8 @@ cd build
 
 # On Mac, do the following:
 MACOSX_DEPLOYMENT_TARGET=10.9 cmake -G Ninja -DCMAKE_BUILD_TYPE=Release \
+    -DLLVM_ENABLE_PROJECTS=clang \
     -DLLVM_ENABLE_ASSERTIONS=NO -DLLVM_ENABLE_THREADS=NO ../llvm/
-time caffeinate ninja clang-format
-strip bin/clang-format
 
 # On Linux, do the following:
 # Note the relative paths that point to your local Chromium checkout.
@@ -76,6 +75,8 @@ cmake -G Ninja -DCMAKE_BUILD_TYPE=Release \
     -DCMAKE_CXX_COMPILER=$PWD/../../chromium/src/third_party/llvm-build/Release+Asserts/bin/clang++ \
     -DCMAKE_ASM_COMPILER=$PWD/../../chromium/src/third_party/llvm-build/Release+Asserts/bin/clang \
     -DLLVM_ENABLE_TERMINFO=OFF -DCMAKE_CXX_STANDARD_LIBRARIES="-static-libgcc -static-libstdc++" ../llvm/
+
+# Finally, build the actual clang-format binary with Ninja
 ninja clang-format
 strip bin/clang-format