Convert commitText("\n") to synthetic Enter key press.

One known WebView app has come to rely on examining KeyDown key code in
JS to detect Enter key presses.  To avoid breaking it, restore the
conversion to KeyDown/KeyUp pair that was removed in
http://crrev.com/1403453002.

BUG=577967

Review URL: https://codereview.chromium.org/1583283008

Cr-Commit-Position: refs/heads/master@{#370284}

content/public/android/java/src/org/chromium/content/browser/input/ImeAdapter.java

@@ -452,8 +452,15 @@ protected void sendSyntheticKeyPress(int keyCode, int flags) {
 
     boolean sendCompositionToNative(CharSequence text, int newCursorPosition, boolean isCommit) {
         if (mNativeImeAdapterAndroid == 0) return false;
-        mViewEmbedder.onImeEvent();
 
+        // One WebView app detects Enter in JS by looking at KeyDown (http://crbug/577967).
+        if (text.equals("\n")) {
+            sendSyntheticKeyPress(KeyEvent.KEYCODE_ENTER,
+                    KeyEvent.FLAG_SOFT_KEYBOARD | KeyEvent.FLAG_KEEP_TOUCH_MODE);
+            return true;
+        }
+
+        mViewEmbedder.onImeEvent();
         long timestampMs = SystemClock.uptimeMillis();
         nativeSendSyntheticKeyEvent(mNativeImeAdapterAndroid, WebInputEventType.RawKeyDown,
                 timestampMs, COMPOSITION_KEY_CODE, 0, 0);

content/public/android/javatests/src/org/chromium/content/browser/input/ImeTest.java

@@ -146,6 +146,24 @@ public void testCommitWhileComposingText() throws Throwable {
         waitAndVerifyStates(3, "hel", 3, 3, -1, -1);
     }
 
+    @SmallTest
+    @Feature({"TextInput", "Main"})
+    public void testCommitEnterKeyWhileComposingText() throws Throwable {
+        focusElementAndWaitForStateUpdate("textarea");
+
+        setComposingText("hello", 1);
+        waitAndVerifyStatesAndCalls(0, "hello", 5, 5, 0, 5);
+
+        // Cancel the current composition and replace it with enter.
+        commitText("\n", 1);
+        // The second new line is not a user visible/editable one, it is a side-effect of Blink
+        // using <br> internally. This only happens when \n is at the end.
+        waitAndVerifyStatesAndCalls(1, "\n\n", 1, 1, -1, -1);
+
+        commitText("world", 1);
+        waitAndVerifyStatesAndCalls(2, "\nworld", 6, 6, -1, -1);
+    }
+
     @SmallTest
     @Feature({"TextInput"})
     public void testImeCopy() throws Exception {