Move CaretBase::shouldRepaintCaret() to FrameCaret.cpp

This patch moves |CaretBase::shouldRepaintCaret()| to "FrameCaret.cpp" because
it is only used in "FrameCaret.cpp" for improving code health.

BUG=n/a
TEST=n/a; no behavior changes

Review-Url: https://codereview.chromium.org/2637953002
Cr-Commit-Position: refs/heads/master@{#444017}
3 files changed