[ObjC ARC] Converts ios/chrome/browser/ui/contextual_search:contextual_search to ARC.

Automatically generated ARCMigrate commit
Notable issues:
 * In ContextualSearchController, there was a block with a weakSelf declared above it and never used. Replaced usage of self in the block with weakself.
* In window_gesture_observer had to suppress a warning, because ARC forbids calling arbitrary selectors.

BUG=624363
TEST=None

Review-Url: https://codereview.chromium.org/2626073003
Cr-Commit-Position: refs/heads/master@{#444717}
20 files changed