gn format: make sure there's a blank line before comments, except at block start

With this, gn format //BUILD.gn results in no changes, other than to
make 'deps += [ "a" ]' turn into one line instead of multiline.

(Child of https://codereview.chromium.org/608593006/)

R=brettw@chromium.org
BUG=348474

Review URL: https://codereview.chromium.org/606123002

Cr-Commit-Position: refs/heads/master@{#297231}

tools/gn/command_format.cc

@@ -72,6 +72,9 @@ class Printer {
   // Remove trailing spaces from the current line.
   void Trim();
 
+  // Whether there's a blank separator line at the current position.
+  bool HaveBlankLine();
+
   // Get the 0-based x position on the current line.
   int CurrentColumn();
 
@@ -141,6 +144,13 @@ void Printer::Trim() {
   output_.resize(n);
 }
 
+bool Printer::HaveBlankLine() {
+  size_t n = output_.size();
+  while (n > 0 && output_[n - 1] == ' ')
+    --n;
+  return n > 2 && output_[n - 1] == '\n' && output_[n - 2] == '\n';
+}
+
 int Printer::CurrentColumn() {
   int n = 0;
   while (n < static_cast<int>(output_.size()) &&
@@ -315,6 +325,15 @@ void Printer::Sequence(SequenceStyle style,
     size_t i = 0;
     for (const auto& x : list) {
       Newline();
+      // If:
+      // - we're going to output some comments, and;
+      // - we haven't just started this multiline list, and;
+      // - there isn't already a blank line here;
+      // Then: insert one.
+      if (i != 0 && x->comments() && !x->comments()->before().empty() &&
+          !HaveBlankLine()) {
+        Newline();
+      }
       ExprStyle expr_style = Expr(x);
       CHECK(!x->comments() || x->comments()->after().empty());
       if (i < list.size() - 1 || style == kSequenceStyleList) {

tools/gn/command_format_unittest.cc

@@ -47,3 +47,4 @@ FORMAT_TEST(015)
 // TODO(scottmg): Requires precedence/parentheses FORMAT_TEST(016)
 FORMAT_TEST(017)
 FORMAT_TEST(018)
+FORMAT_TEST(019)

tools/gn/format_test_data/019.gn

+# Make sure blank lines are maintained before comments in lists.
+
+deps = [
+      "//pdf",  # Not compiled on Android in GYP yet, either.
+      "//ppapi:ppapi_c",
+      "//third_party/libusb",
+      "//ui/keyboard",  # Blocked on content.
+
+      # Seems to not be compiled on Android. Otherwise it will need a config.h.
+      "//third_party/libxslt",
+
+      # Not relevant to Android.
+      "//ash",
+      "//tools/gn",
+
+      # Multiple line
+      # comment
+      # here.
+      "//ui/aura",
+      "//ui/display",
+      "//ui/views",
+      "//ui/views/controls/webview",
+]

tools/gn/format_test_data/019.golden

+# Make sure blank lines are maintained before comments in lists.
+
+deps = [
+  "//pdf",  # Not compiled on Android in GYP yet, either.
+  "//ppapi:ppapi_c",
+  "//third_party/libusb",
+  "//ui/keyboard",  # Blocked on content.
+
+  # Seems to not be compiled on Android. Otherwise it will need a config.h.
+  "//third_party/libxslt",
+
+  # Not relevant to Android.
+  "//ash",
+  "//tools/gn",
+
+  # Multiple line
+  # comment
+  # here.
+  "//ui/aura",
+  "//ui/display",
+  "//ui/views",
+  "//ui/views/controls/webview",
+]