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This CL makes the feature_compiler support non-default toolchains by
avoiding assuming that the gen directory is always gen/.

Bug: 1129223
Change-Id: I2a361cbb1ce4a220fca03c8db6457f2fb03d81d7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2419815
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/master@{#808636}