[ios] Fix DIR_SOURCE_ROOT for EarlGrey2 tests

On iOS, the application has no access to source_root directly as
all applications are bundled. Thus the test data are bundled as
asserts and need to be located in DIR_ASSETS.

For most of the tests, the directory containing the app binary
is the directory containing the assets, but this is not the case
for EarlGrey2 test that are part of a PlugIns bundle.

Using DIR_ASSETS instead loads the correct path since it is
relative to the code that is running, not to the main application.

Bug: none
Change-Id: I25b0c7b9ac8a7686332591f4488440ffb7f5d4c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2149427
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Mark Mentovai <mark@chromium.org>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759233}

base/base_paths_mac.mm

@@ -82,6 +82,11 @@ bool PathProviderMac(int key, base::FilePath* result) {
       return success;
     }
     case base::DIR_SOURCE_ROOT:
+#if defined(OS_IOS)
+      // On iOS, there is no access to source root, however, the necessary
+      // resources are packaged into the test as assets.
+      return PathService::Get(base::DIR_ASSETS, result);
+#else
       // Go through PathService to catch overrides.
       if (!PathService::Get(base::FILE_EXE, result))
         return false;
@@ -89,7 +94,6 @@ bool PathProviderMac(int key, base::FilePath* result) {
       // Start with the executable's directory.
       *result = result->DirName();
 
-#if !defined(OS_IOS)
       if (base::mac::AmIBundled()) {
         // The bundled app executables (Chromium, TestShell, etc) live five
         // levels down, eg:
@@ -100,7 +104,7 @@ bool PathProviderMac(int key, base::FilePath* result) {
         // src/xcodebuild/{Debug|Release}/base_unittests
         *result = result->DirName().DirName();
       }
-#endif
+#endif  // !defined(OS_IOS)
       return true;
     case base::DIR_USER_DESKTOP:
 #if defined(OS_IOS)
@@ -112,7 +116,7 @@ bool PathProviderMac(int key, base::FilePath* result) {
 #endif
     case base::DIR_ASSETS:
 #if defined(OS_IOS)
-      // TODO(https://crbug.com/957792): Assets live alongside the executable.
+      // On iOS, the assets are located next to the module binary.
       return PathService::Get(base::DIR_MODULE, result);
 #else
       if (!base::mac::AmIBundled()) {