## 🐛 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

* <p>
   * More exactly, this method will return the value of the property with the given name, if it
   * exists. If it does not, then the value of the attribute with the given name is returned. If
   * neither exists, null is returned.
   * <p>
   * The "style" attribute is converted as best can be to a text representation with a trailing
   * semi-colon.
   * <p>
   * The following are deemed to be "boolean" attributes, and will return either "true" or null:
   * <p>
   * async, autofocus, autoplay, checked, compact, complete, controls, declare, defaultchecked,
   * defaultselected, defer, disabled, draggable, ended, formnovalidate, hidden, indeterminate,
   * iscontenteditable, ismap, itemscope, loop, multiple, muted, nohref, noresize, noshade,
   * novalidate, nowrap, open, paused, pubdate, readonly, required, reversed, scoped, seamless,
   * seeking, selected, truespeed, willvalidate
   * <p>
   * Finally, the following commonly mis-capitalized attribute/property names are evaluated as
   * expected:
   * <ul>
   * <li>If the given name is "class", the "className" property is returned.
   * <li>If the given name is "readonly", the "readOnly" property is returned.
   * </ul>
   * <i>Note:</i> The reason for this behavior is that users frequently confuse attributes and
   * properties. If you need to do something more precise, use {@link #getDomAttribute(String)}
   * or {@link #getDomProperty(String)} to obtain the result you desire.
   * <p>
   * See <a href="https:w3c.github.io/webdriver/#get-element-attribute">W3C WebDriver specification</a>
   * for more details.
   *
   * @param name The name of the attribute.