[Editing][BugFix] Fix if condition in ReplaceSelectionCommand

[Editing][BugFix] Fix if condition in ReplaceSelectionCommand

In ReplaceSelectionCommand::doApply, we run exec BreakBlockquoteCommand to break a Mail
 block quote and assume BR element is inserted at endingSelection.start. Then we remove the BR
 element. This command is run if selection is start in a mail blockquote element.
However, selection start can be modified before L1006 because of deleting selection
 to prepare for insertion.

This CL checks the condition again.
Plus, refactoring code flow.

BUG=534617
Review URL: https://codereview.chromium.org/1395693009

Cr-Commit-Position: refs/heads/master@{#355497}
(cherry picked from commit 41f2fa33e2126633ba36e61eed9abb365990bd49)

Committed: https://chromium.googlesource.com/chromium/src/+/761db0fc20eaf195e66909f904a317b18b493615

[yoichio, 2015-10-30 07:00:21]

Committed patchset #1 (id:1) manually as
761db0fc20eaf195e66909f904a317b18b493615 (tree was closed).