Simplify PastePopupMenu delegate interface and layout attributes

Remove redundant calls to canPaste() in PastePopupMenu. Inhibit calls to show
PastePopupMenu if clipboard is empty. Removed unwanted layout attributes to
fix the greying of the PastePopupMenu when diplayed beside the insertion
handler.

BUG=400705

Review URL: https://codereview.chromium.org/449033002

Cr-Commit-Position: refs/heads/master@{#288973}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288973 0039d316-1c4b-4281-b951-d872f2087c98

• content/public/android/java/src/org/chromium/content/browser/ContentViewCore.java[diff]
• content/public/android/java/src/org/chromium/content/browser/input/PastePopupMenu.java[diff]