Remove Windows native textfield and Omnibox code.

Remove native_textfield_win.* and omnibox_view_win.*
(off-by-default since M-30, not supported on Win Aura)
Remove omnibox_views.* helper file, inline remaining uses.
Remove the about:flags entry, flags, and strings.
Remove the Textfield::IsViewsTextfieldEnabled() helper.
Remove native control implementation-specific code.
(FindBarView/BrowserView/LocationBarView/Textfield/etc.)
Remove OmniboxView::AddToView and overrides.
Refactor LocationBarView::omnibox_view_ and related code.
Reduce SkipDefaultKeyEventProcessing code duplication.
Minor refactoring and other cleanup.

Here are some cleanup tasks for followup CLs:
Issue 319037: Rename LocationBar::GetLocationEntry to GetOmniboxView.
Issue 319035: Investigate BrowserView::DoCutCopyPaste necessity and cleanup.
Issue 319038: Remove DialogDelegate::UseNewStyle.
Issue 319039: Cleanup ui/base/win/extra_sdk_defines.h
Issue 319042: Consolidate remaining Views Textfield and Omnibox code.

BUG=131660
TEST=No build breaks or regressions, --disable-views-textfield and use of the native Windows textfields/Omnibox are no longer available.
R=pkasting@chromium.org,sky@chromium.org

Review URL: https://codereview.chromium.org/68253007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235220 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235220