Add a flag for pattern initialization

Add the flag so we can turn on pattern initialization from
https://reviews.llvm.org/D54604 in the future.

Bug: 966551
Change-Id: I631bd71f5e1da553ec7a9e480537adc8573d1047
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1835027
Commit-Queue: Jian Cai <jiancai@google.com>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#721174}
1 file changed