gn format: properly break and align multiple suffix comments

Previously

  a = [
    "x",
    "y",   # Comment1
           # Comment2
  ]

would turn into
  a = [
    "x",
    "y",   # Comment1# Comment2
  ]

(Child of https://codereview.chromium.org/606123002/)

R=brettw@chromium.org
BUG=348474

Review URL: https://codereview.chromium.org/613473002

Cr-Commit-Position: refs/heads/master@{#297235}

tools/gn/command_format.cc

@@ -122,14 +122,22 @@ void Printer::Newline() {
   if (!comments_.empty()) {
     Print("  ");
     int i = 0;
+    // Save the margin, and temporarily set it to where the first comment
+    // starts so that multiple suffix comments are vertically aligned. This
+    // will need to be fancier once we enforce 80 col.
+    int old_margin = margin_;
     for (const auto& c : comments_) {
-      if (i > 0) {
+      if (i == 0)
+        margin_ = CurrentColumn();
+      else {
         Trim();
         Print("\n");
         PrintMargin();
       }
       TrimAndPrintToken(c);
+      ++i;
     }
+    margin_ = old_margin;
     comments_.clear();
   }
   Trim();

tools/gn/command_format_unittest.cc

@@ -48,3 +48,4 @@ FORMAT_TEST(015)
 FORMAT_TEST(017)
 FORMAT_TEST(018)
 FORMAT_TEST(019)
+FORMAT_TEST(020)

tools/gn/format_test_data/020.gn

+cflags = [
+  "/wd4267",  # size_t -> int.
+  "/wd4324",  # Structure was padded due to __declspec(align()), which is
+              # uninteresting.
+]

tools/gn/format_test_data/020.golden

+cflags = [
+  "/wd4267",  # size_t -> int.
+  "/wd4324",  # Structure was padded due to __declspec(align()), which is
+              # uninteresting.
+]