Add .externalToolBuilders/ to .gitignore.
Add .externalToolBuilders/ to .gitignore.

This directory contains some Eclipse settings.

Review URL: https://codereview.chromium.org/1210073012

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#336867}

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