Provide GN support for cachetool and content_decoder_tool

This CL also make the cachetool linux only as content_decoder_tool
since it is not used anywhere else.

BUG=582080

Review URL: https://codereview.chromium.org/1838443002

Cr-Commit-Position: refs/heads/master@{#383741}
3 files changed