Revert "Revert "find_headers.py to better find headers.""

This reverts commit 59da548b0c4d4239e0ec1855d3f7f77a2bff4b93.

This now provides more information on failure and always sets the
source root for cases where the output directory is not a subdirectory
of the source directory.

Change-Id: I8e317814d4686d9a0736b7097b404920fa55e769
Reviewed-on: https://skia-review.googlesource.com/130134
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
3 files changed