Tool added to extract network traffic annotations.

tools/clang/traffic_annotation_extractor/README.txt

Index: tools/clang/traffic_annotation_extractor/README.txt
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+This is a clang tool to extract network traffic annotations. The tool is run by
+tools/traffic_annotation/traffic_annotaion_auditor. Refer to it for help on how
+to use.
+
+Build on Linux, run:
+tools/clang/scripts/update.py --bootstrap --force-local-build
+  --without-android --extra-tools blink_gc_plugin plugins
+  rewrite_to_chrome_style traffic_annotation_extractor
+
+Build on Window:
+1. Either open a "VS2015 x64 Native Tools Command Prompt", or open a normal
+   command prompt and run "depot_tools\win_toolchain\vs_fies\
+   $long_autocompleted_hash\win_sdk\bin\setenv.cmd /x64"
+2. Run python tools/clang/scripts/update.py --bootstrap --force-local-build
+   --without-android --extra-tools blink_gc_plugin plugins
+   rewrite_to_chrome_style traffic_annotation_extractor
+