Fix Mac text selection behavior to collapse selection when reversing back to anchor point

This commit modifies the behavior of Textfield line selection commands (Command+Shift+Left/Right) on macOS to match native platform conventions (e.g., latest NSTextView, TextEdit). Previously, using selection extension commands to reverse back to the anchor point would "flip" the selection and extend in the opposite direction (due to gfx::SELECTION_EXTEND blindly flipping the anchor when crossed). Native macOS behavior is to collapse (cancel) the selection when the cursor returns to the anchor point. Changes include: adding IS_MAC platform-specific logic in ui/gfx/render_text.cc's MoveCursor method using strict inequality to determine selection reversal, ensuring returning exactly to the selection start collapses rather than reverses; updating the SELECTION_CARET branch to handle caret at start position; correspondingly updating tests in render_text_unittest.cc, bridged_native_widget_unittest.mm, textfield_unittest.cc, and textarea_unittest.cc.