[ChromeDriver] Fix mouse move after click

In a sequence of mouse pointerDown, pointerUp, and pointerMove actions,
the mouse button saved by the first two actions needs to be cleared
while processing the last action, as no button is pressed.

Fixes the following Selenium Java test:
CombinedInputActionsTest.testControlClickingOnMultiSelectionList

Bug: chromedriver:1897
Change-Id: I0bd7b377fa0c977c8c20c5c0e306f772ee30ca98
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1547130
Auto-Submit: John Chen <johnchen@chromium.org>
Reviewed-by: Lan Wei <lanwei@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646444}

chrome/test/chromedriver/window_commands.cc

@@ -1361,6 +1361,8 @@ Status ExecutePerformActions(Session* session,
           if (action_type == "pointerDown" || action_type == "pointerUp") {
             pointer_action->GetString("button", &button_type);
             click_count = 1;
+          } else if (buttons == 0) {
+            button_type.clear();
           }
           MouseEvent event(StringToMouseEventType(action_type),
                            StringToMouseButton(button_type), x, y, 0, buttons,