Use static_cast instead of reinterpret_cast for downcasting

static_cast is better suited for downcasting since it can check that the
types are related.

/chrome/browser/ui/views/extensions
This CL was uploaded by git cl split.

R=rdevlin.cronin@chromium.org

Change-Id: I5300e111b30d94c6571f090128a32ad638f0f2b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2435980
Auto-Submit: Kevin McNee <mcnee@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811400}