## 🐛 Bug Report

When using `WebElement#getCssValue` the returned value is not necessarily a rgba value when the requested color is in fact an hexadecimal value, contrarily to what's explained in the WebElement javadoc:
https://github.com/SeleniumHQ/selenium/blob/master/java/client/src/org/openqa/selenium/WebElement.java#L246-L265

Now I've not been able to track down in the W3C WebDriver specification if the color values should be systematically rgba or not, so maybe it's just a javadoc issue.

## To Reproduce

Detailed steps to reproduce the behavior:
Request the CSS color value of an element containing an hexadecimal color.

## Expected behavior

The color is returned as an RGBA value.

## Test script or set of commands reproducing this issue

```
WebDriver driver = new FirefoxDriver();
driver.get("http://www.google.com");
// The "connection" button, cf the following gif
WebElement element = driver.findElement(By.id("gb_70"));
// expected to be RGBA according to the javadoc
String color = element.getCssValue("border-top-color");
System.out.println("Color CSS value: " + color);
driver.quit();
```

![google-button-border-color](https://user-images.githubusercontent.com/1478232/67192046-1b2c4780-f3f3-11e9-833a-ccd4b410f56a.gif)

## Environment

OS: Linux
Browser: Firefox
Browser version: 69.0.3
Browser Driver version: GeckoDriver 0.25.0
Language Bindings version: Java 3.141.59

* @throws IllegalArgumentException if keysToSend is null
   */
  void sendKeys(CharSequence... keysToSend);
  /**
   * If this element is a form entry element, this will reset its value.
   * <p>
   * See <a href="https:w3c.github.io/webdriver/#element-clear">W3C WebDriver specification</a>
   * and <a href="https:html.spec.whatwg.org/#concept-form-reset-control">HTML specification</a>
   * for more details.
   */
  void clear();
  /**
   * Get the tag name of this element. <b>Not</b> the value of the name attribute: will return
   * <code>"input"</code> for the element <code>&lt;input name="foo" /&gt;</code>.
   * <p>
   * See <a href="https:w3c.github.io/webdriver/#get-element-tag-name">W3C WebDriver specification</a>
   * for more details.
   *
   * @return The tag name of this element.
   */
  String getTagName();
  /**
   * Get the value of the given property of the element. Will return the current value, even if
   * this has been modified after the page has been loaded.
   * <p>
   * See <a href="https:w3c.github.io/webdriver/#get-element-property">W3C WebDriver specification</a>
   * for more details.
   *
   * @param name The name of the property.
   * @return The property's current value or null if the value is not set.