Main content extraction proto convertor unittests

Creating/updating an expected proto output.

Turn on WRITE_DEBUG_PROTO in main_content_extractor_proto_convertor_unittest.cc.
Run its services unittests under the suite of MainContentExtractorProtoConvertorTest. An example command line is:
console
```
out/Debug/services_unittests --gtest_filter=MainContentExtractorProtoConvertor*
```

Run the following gqui command to convert a debug binary proto into an expected text proto:

console

gqui from rawproto:[DEBUG PROTO PATH] proto screenai.ViewHierarchy --noprotoprint_annotations --outfile=textproto:[OUTPUT TEXTPROTO PATH]

Delete the most outer curly brackets in the text proto.
Rename the text proto based on kProtoConversionSampleExpectedFileNameFormat in main_content_extractor_proto_convertor_unittest.cc.
Move the text proto to services/test/data/screen_ai.