Support ability to specify custom FF bin location (#2846)

Fixes #2845

Currently even though FirefoxDriver has the mechanisms
to accept a custom location of firefox binary, the 
location is not honoured by GeckoDriverService.
Fixing this by passing the location to geckodriver

Author: krmahadevan

java/client/src/org/openqa/selenium/firefox/FirefoxDriver.java

@@ -255,7 +255,12 @@ private static final CommandExecutor createCommandExecutor(Capabilities desiredC
     if (isLegacy(desiredCapabilities)) {
       return new LazyCommandExecutor(binary, profile);
     }
-    GeckoDriverService.Builder builder = new GeckoDriverService.Builder();
+    GeckoDriverService.Builder builder;
+    if (binary == null) {
+      builder = new GeckoDriverService.Builder();
+    } else {
+      builder = new GeckoDriverService.Builder(binary);
+    }
     builder.usingPort(0);
     return new DriverCommandExecutor(builder.build());
   }

java/client/src/org/openqa/selenium/firefox/GeckoDriverService.java

@@ -25,7 +25,6 @@
 import org.openqa.selenium.WebDriverException;
 import org.openqa.selenium.net.PortProber;
 import org.openqa.selenium.firefox.internal.Executable;
-import org.openqa.selenium.os.CommandLine;
 import org.openqa.selenium.remote.service.DriverService;
 
 import java.io.File;
@@ -79,6 +78,18 @@ protected void waitUntilAvailable() throws MalformedURLException {
   public static class Builder extends DriverService.Builder<
     GeckoDriverService, GeckoDriverService.Builder> {
 
+    private Executable binary;
+    public Builder() {
+      this(new FirefoxBinary());
+    }
+
+    /**
+     * @param binary - A custom location where the Firefox binary is available.
+     */
+    public Builder(FirefoxBinary binary) {
+      this.binary = binary.getExecutable();
+    }
+
     @Override
     protected File findDefaultExecutable() {
       return findExecutable(
@@ -95,9 +106,8 @@ protected ImmutableList<String> createArgs() {
         argsBuilder.add(String.format("--log-file=\"%s\"", getLogFile().getAbsolutePath()));
       }
       try {
-        Executable firefoxExe = new Executable(null);
         argsBuilder.add("-b");
-        argsBuilder.add(firefoxExe.getPath());
+        argsBuilder.add(binary.getPath());
       } catch (WebDriverException e) {
         // Unable to find Firefox. GeckoDriver will be responsible for finding
         // Firefox on the PATH or via a capability.