Make SelectionModifier::Modify() to do nothing for empty selection

This patch makes |SelectionModifier::Modify()| to do nothing for empty selection
to avoid calling |NextWordPosition()| with null-position.

Note: |NextWordPosition()| doesn't accept null-position since the patch[1].

[1] http:/crrev.com/c/1004536 Make NextWordPosition() to utilize
TextOffsetMapping

Bug: 832061
Change-Id: I534af079f068f709c9b48a408988cb413a520b86
Reviewed-on: https://chromium-review.googlesource.com/1014841Reviewed-by: Xiaocheng Hu <xiaochengh@chromium.org>
Commit-Queue: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551539}

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

@@ -639,6 +639,8 @@ bool SelectionModifier::Modify(SelectionModifyAlteration alter,
       GetFrame().GetDocument()->Lifecycle());
 
   selection_ = PrepareToModifySelection(alter, direction);
+  if (selection_.IsNone())
+    return false;
 
   bool was_range = selection_.IsRange();
   VisiblePosition original_start_position = selection_.VisibleStart();

third_party/blink/renderer/core/editing/selection_modifier.h

@@ -28,6 +28,7 @@
 #define THIRD_PARTY_BLINK_RENDERER_CORE_EDITING_SELECTION_MODIFIER_H_
 
 #include "base/macros.h"
+#include "third_party/blink/renderer/core/core_export.h"
 #include "third_party/blink/renderer/core/editing/selection_template.h"
 #include "third_party/blink/renderer/core/editing/visible_selection.h"
 #include "third_party/blink/renderer/platform/layout_unit.h"
@@ -41,7 +42,7 @@ enum class SelectionModifyAlteration { kMove, kExtend };
 enum class SelectionModifyVerticalDirection { kUp, kDown };
 enum class SelectionModifyDirection { kBackward, kForward, kLeft, kRight };
 
-class SelectionModifier {
+class CORE_EXPORT SelectionModifier {
   STACK_ALLOCATED();
 
  public:

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

@@ -11,6 +11,15 @@ namespace blink {
 
 class SelectionModifierTest : public EditingTestBase {};
 
+TEST_F(SelectionModifierTest, ExtendForwardByWordNone) {
+  SetBodyContent("abc");
+  SelectionModifier modifier(GetFrame(), SelectionInDOMTree());
+  modifier.Modify(SelectionModifyAlteration::kExtend,
+                  SelectionModifyDirection::kForward, TextGranularity::kWord);
+  // We should not crash here. See http://crbug.com/832061
+  EXPECT_EQ(SelectionInDOMTree(), modifier.Selection().AsSelection());
+}
+
 TEST_F(SelectionModifierTest, leftPositionOf) {
   const char* body_content =
       "<b id=zero>0</b><p id=host><b id=one>1</b><b id=two>22</b></p><b "
@@ -49,6 +58,15 @@ TEST_F(SelectionModifierTest, leftPositionOf) {
                 .DeepEquivalent());
 }
 
+TEST_F(SelectionModifierTest, MoveForwardByWordNone) {
+  SetBodyContent("abc");
+  SelectionModifier modifier(GetFrame(), SelectionInDOMTree());
+  modifier.Modify(SelectionModifyAlteration::kMove,
+                  SelectionModifyDirection::kForward, TextGranularity::kWord);
+  // We should not crash here. See http://crbug.com/832061
+  EXPECT_EQ(SelectionInDOMTree(), modifier.Selection().AsSelection());
+}
+
 TEST_F(SelectionModifierTest, rightPositionOf) {
   const char* body_content =
       "<b id=zero>0</b><p id=host><b id=one>1</b><b id=two>22</b></p><b "