Toggling caret browsing should invalidate style.

When caret browsing is enabled it should cause the cursor to start
blinking if it wasn't already. To do this, it's sufficient to just
invalidate style. This results in calling
FrameCaret::UpdateStyleAndLayoutIfNeeded() which starts the caret
timer if needed.

Bug: 1116658
Change-Id: I55a249878a1646a358a584e6ffccd501bfdd8189
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2357963
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Kent Tamura <tkent@chromium.org>
Auto-Submit: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798961}

third_party/blink/renderer/core/editing/frame_caret_test.cc

@@ -104,6 +104,7 @@ TEST_F(FrameCaretTest, ShouldBlinkCaretWhileCaretBrowsing) {
   Selection().SetCaretVisible(true);
   EXPECT_FALSE(ShouldBlinkCaret(caret));
   GetDocument().GetFrame()->GetSettings()->SetCaretBrowsingEnabled(true);
+  UpdateAllLifecyclePhasesForTest();
   EXPECT_TRUE(ShouldBlinkCaret(caret));
 }
 

third_party/blink/renderer/core/frame/settings.json5

@@ -454,6 +454,7 @@
     {
       name: "caretBrowsingEnabled",
       initial: false,
+      invalidate: "Style",
     },
 
     // Font scale factor for accessibility, applied as part of text autosizing.