Tool added to extract network traffic annotations.

tools/clang/traffic_annotation_extractor/README.md

Index: tools/clang/traffic_annotation_extractor/README.md
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+# Traffic Annotation Extrator
+This is a clang tool to extract network traffic annotations. The tool is run by
+`tools/traffic_annotation/auditor/traffic_annotaion_auditor.cc`. Refer to it for
+help on how to use.
+
+## Build on Linux
+`tools/clang/scripts/update.py --bootstrap --force-local-build
+   --without-android --extra-tools 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_files\
+   $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 traffic_annotation_extractor`
+
+## Usage
+Run `traffic_annotation_extractor --help` for parameters help.
+The executable extracts network traffic annotations from given file paths based
+  on build parameters in build path, and writes them as separate files into an
+  output directory.
+Each output file will have the following format:
+  - Line 1: File path.
+  - Line 2: Name of the function in which annotation is defined.
+  - Line 3: Line number of annotation.
+  - Line 4: Unique id of annotation.
+  - Line 5-: Serialized protobuf of the annotation.