Add sticky focus mechanism into ui::InputMethod

Currently there are two callers of ui::InputMethod::SetFocusedTextInputClient.
One is RenderWidgetHostViewAura and the other is ui::views::InputMethodBridge.
This makes it hard to implement ui::InputMethod correctly because these
callers work independently to each other.

As a tentative solution, this CL introduces "sticky" focus mechanism to solve
this problem. It will be used as follows in the next CL.

 1. RWHVA calls SetStickyFocusedTextInputClient(this)
    -> "FocusedTextInputClient" is locked to RWHVA
 2. InputMethodBridge calls SetFocusedTextInputClient(this)
    -> This will be ignored because RWHVA already locked the focus.
 3. RWHVA calls SetStickyFocusedTextInputClient(NULL)
    -> "FocusedTextInputClient" is unlocked
 4. InputMethodBridge calls SetFocusedTextInputClient(this)
    -> "FocusedTextInputClient" is set to InputMethodBridge 

In short, InputMethodBridge can gain text input focus as long as RWHVA does
not own the input focus. This special rule solves the problem.

In a long term solution, @nona is working on an unified text input client
focus manager, which is expected to replace this sticky focus mechanism.
I hope it will happen near future.

This CL also fixes a minor issue that
InputMethodBase::On{Will/Did}ChangeFocusedClient are called redundantly when
InputMethod::SetFocusedTextInputClient is called twice or more.
Except for this minor fix, there CL never changes the production code.

BUG=287620
TEST=Ran unit tests. Manually done with/without --enable-text-services-framework on Aura/non-Aura Windows 7.

Review URL: https://chromiumcodereview.appspot.com/23754015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@223054 0039d316-1c4b-4281-b951-d872f2087c98

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