AW: add gitignore for support lib boundary interfaces

No change to logic.

This adds a gitignore file to ignore *.iml files, which are created by
Android Studio. We add the rule at this level so that it gets picked up
when we mirror this folder into Android (same reason we have proguard
rules at this specific level).

This is not expected to affect Chromium (other than it ignores *.iml
files in this subtree, which should not cause issues for developers).

Based off sergeyv@'s CL http://ag/4570535.

R=torne@chromium.org

Change-Id: I015e29269923054afffb9288c37170cc124be7c2
Reviewed-on: https://chromium-review.googlesource.com/1142368Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576170}

android_webview/support_library/boundary_interfaces/.gitignore

+# For Android Studio, when we mirror this repo into the Android tree
+*.iml