Deduplicating code |WebContentsDelegateAndroid::ShowContextMenu()|

Clank, webview, and content shell had duplicate code for showing the
paste popup menu in |ShowContextMenu()|. This CL de-duped it.

BUG=627234

Review-Url: https://codereview.chromium.org/2724533002
Cr-Commit-Position: refs/heads/master@{#453810}
6 files changed