[iOS] Support arrow keys when displaying inline autocomplete.

Adds support for arrows left/right when displaying inline autocomplete.
Arrow right accepts the input and moves the cursor to the end of the
textfield; arrow left accepts the input and keeps the cursor in the
same position - mirroring what desktop does.

Bug: 822307
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I58608e6cb41a904ffb12890e238a4ca000cf5b6f
Reviewed-on: https://chromium-review.googlesource.com/973525
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545082}

ios/chrome/browser/ui/omnibox/omnibox_text_field_ios.mm

@@ -704,7 +704,7 @@ NSString* const kOmniboxFadeAnimationKey = @"OmniboxFadeAnimation";
 
 - (NSArray<UIKeyCommand*>*)keyCommands {
   NSMutableArray<UIKeyCommand*>* commands = [[self upDownCommands] mutableCopy];
-  if ([self isPreEditing]) {
+  if ([self isPreEditing] || [self hasAutocompleteText]) {
     [commands addObjectsFromArray:[self leftRightCommands]];
   }
 
@@ -719,10 +719,12 @@ NSString* const kOmniboxFadeAnimationKey = @"OmniboxFadeAnimation";
   [self.suggestionCommandsEndpoint highlightPreviousSuggestion];
 }
 
-#pragma mark preedit-only key commands
+#pragma mark preedit and inline autocomplete key commands
 
 // React to left and right keys when in preedit state to exit preedit and put
-// cursor to the beginning/end of the textfield.
+// cursor to the beginning/end of the textfield; or if there is inline
+// suggestion displayed, accept it and put the cursor before/after the
+// suggested text.
 - (NSArray<UIKeyCommand*>*)leftRightCommands {
   UIKeyCommand* commandLeft =
       [UIKeyCommand keyCommandWithInput:UIKeyInputLeftArrow
@@ -737,22 +739,57 @@ NSString* const kOmniboxFadeAnimationKey = @"OmniboxFadeAnimation";
 }
 
 - (void)keyCommandLeft {
-  [self exitPreEditState];
+  DCHECK([self isPreEditing] || [self hasAutocompleteText]);
+
+  // Cursor offset.
+  NSInteger offset = 0;
+  if ([self isPreEditing]) {
+    [self exitPreEditState];
+  }
+
+  if ([self hasAutocompleteText]) {
+    // The cursor should stay in the end of the user input.
+    offset = self.text.length;
+
+    // Accept autocomplete suggestion.
+    [self acceptAutocompleteText];
+  }
+
   UITextPosition* beginning = self.beginningOfDocument;
+  UITextPosition* cursorPosition =
+      [self positionFromPosition:beginning offset:offset];
   UITextRange* textRange =
-      [self textRangeFromPosition:beginning toPosition:beginning];
-
+      [self textRangeFromPosition:cursorPosition toPosition:cursorPosition];
   self.selectedTextRange = textRange;
 }
 
 - (void)keyCommandRight {
-  [self exitPreEditState];
+  DCHECK([self isPreEditing] || [self hasAutocompleteText]);
+
+  if ([self isPreEditing]) {
+    [self exitPreEditState];
+  }
+
+  if ([self hasAutocompleteText]) {
+    [self acceptAutocompleteText];
+  }
+
+  // Put the cursor to the end of the input.
   UITextPosition* end = self.endOfDocument;
   UITextRange* textRange = [self textRangeFromPosition:end toPosition:end];
 
   self.selectedTextRange = textRange;
 }
 
+// A helper to accept the current autocomplete text.
+- (void)acceptAutocompleteText {
+  DCHECK([self hasAutocompleteText]);
+  // Strip attributes and set text as if the user typed it.
+  NSAttributedString* string =
+      [[NSAttributedString alloc] initWithString:_selection.text];
+  [self setText:string userTextLength:string.length];
+}
+
 #pragma mark - helpers
 
 // Gets the bounds of the rect covering the URL.