[chromedriver] Change the search order of Chrome binary on Linux

Ideally, we should respect users' PATH when searching for Chrome binary.
For now, change the order of search paths to a better approximation of
the most common defaults across Linux distros.

In particular, try Chrome's default installation location AT LAST, so
that symlinks created by tools like update-alternatives are tried first,
which means honoring users' preferences of Chrome channels.

Change-Id: Ic77e439d3a3b428194af45b38bb85cfbb8a97508
Reviewed-on: https://chromium-review.googlesource.com/965444Reviewed-by: Jonathon Kereliuk <kereliuk@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543696}

chrome/test/chromedriver/chrome/chrome_finder.cc

@@ -47,13 +47,16 @@ void GetApplicationDirs(std::vector<base::FilePath>* locations) {
 }
 #elif defined(OS_LINUX)
 void GetApplicationDirs(std::vector<base::FilePath>* locations) {
-  locations->push_back(base::FilePath("/opt/google/chrome"));
-  locations->push_back(base::FilePath("/usr/local/bin"));
+  // TODO: Respect users' PATH variables.
+  // Until then, we use an approximation of the most common defaults.
   locations->push_back(base::FilePath("/usr/local/sbin"));
-  locations->push_back(base::FilePath("/usr/bin"));
+  locations->push_back(base::FilePath("/usr/local/bin"));
   locations->push_back(base::FilePath("/usr/sbin"));
-  locations->push_back(base::FilePath("/bin"));
+  locations->push_back(base::FilePath("/usr/bin"));
   locations->push_back(base::FilePath("/sbin"));
+  locations->push_back(base::FilePath("/bin"));
+  // Lastly, try the default installation location.
+  locations->push_back(base::FilePath("/opt/google/chrome"));
 }
 #elif defined(OS_ANDROID)
 void GetApplicationDirs(std::vector<base::FilePath>* locations) {