Make sure there is no crash on parsing empty manifest

Changes match the logic in SourceHighlighter in the same file and will
result in UI showing message akin to 'No source file available'. This is fine
as already the same happens for other source files in bundled extensions.

BUG=627896

Review-Url: https://codereview.chromium.org/2245143004
Cr-Commit-Position: refs/heads/master@{#412556}

extensions/browser/file_highlighter.cc

@@ -113,8 +113,10 @@ ManifestHighlighter::ManifestHighlighter(const std::string& manifest,
                                          const std::string& key,
                                          const std::string& specific)
     : FileHighlighter(manifest) {
-  start_ = contents_.find('{') + 1;
+  start_ = contents_.find('{');
+  start_ = start_ == std::string::npos ? contents_.size() : start_ + 1;
   end_ = contents_.rfind('}');
+  end_ = end_ == std::string::npos ? contents_.size() : end_;
   Parse(key, specific);
 }
 
@@ -125,11 +127,11 @@ ManifestHighlighter::~ManifestHighlighter() {
 void ManifestHighlighter::Parse(const std::string& key,
                                 const std::string& specific) {
   // First, try to find the bounds of the full key.
-  if (FindBounds(key, true) /* enforce at top level */ ) {
+  if (FindBounds(key, true) /* enforce at top level */) {
     // If we succeed, and we have a specific location, find the bounds of the
     // specific.
     if (!specific.empty())
-      FindBounds(specific, false /* don't enforce at top level */ );
+      FindBounds(specific, false /* don't enforce at top level */);
 
     // We may have found trailing whitespace. Don't use base::TrimWhitespace,
     // because we want to keep any whitespace we find - just not highlight it.

extensions/browser/file_highlighter_unittest.cc

@@ -93,6 +93,11 @@ TEST(ManifestHighlighterUnitTest, ManifestHighlighterUnitTest) {
   ManifestHighlighter international_feature(
       kManifest, "international_key", std::string());
   EXPECT_EQ(kInternationalFeature, international_feature.GetFeature());
+
+  // Empty manifest. Check that there is no crash.
+  const char kEmptyManifest[] = "";
+  ManifestHighlighter no_feature(kEmptyManifest, std::string(), std::string());
+  EXPECT_EQ(std::string(), no_feature.GetFeature());
 }
 
 TEST(SouceHighlighterUnitTest, SourceHighlighterUnitTest) {