[IntersectionObserver] Fix some edge cases in parsing options

BUG=

Change-Id: Ia3b6dc5f8dfd6036a55a2dd91e76b9c94d6f9535
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2388706Reviewed-by: vmpstr <vmpstr@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803904}

third_party/blink/renderer/core/intersection_observer/intersection_observer.cc

@@ -89,6 +89,7 @@ void ParseMargin(String margin_parameter,
   CSSTokenizer tokenizer(margin_parameter);
   const auto tokens = tokenizer.TokenizeToEOF();
   CSSParserTokenRange token_range(tokens);
+  token_range.ConsumeWhitespace();
   while (token_range.Peek().GetType() != kEOFToken &&
          !exception_state.HadException()) {
     if (margin.size() == 4) {
@@ -136,6 +137,9 @@ void ParseThresholds(const DoubleOrDoubleSequence& threshold_parameter,
       thresholds.push_back(base::MakeClampedNum<float>(threshold_value));
   }
 
+  if (thresholds.IsEmpty())
+    thresholds.push_back(0.f);
+
   for (auto threshold_value : thresholds) {
     if (std::isnan(threshold_value) || threshold_value < 0.0 ||
         threshold_value > 1.0) {

third_party/blink/web_tests/external/wpt/intersection-observer/observer-attributes.html

@@ -15,6 +15,15 @@ test(function() {
   test(function() { assert_equals(observer.rootMargin, "0px 0px 0px 0px") },
        "observer.rootMargin");
 
+  observer = new IntersectionObserver(function(e) {}, {
+    rootMargin: " ",
+    threshold: []
+  });
+  test(function() { assert_array_equals(observer.thresholds, [0]) },
+       "empty observer.thresholds");
+  test(function() { assert_equals(observer.rootMargin, "0px 0px 0px 0px") },
+       "whitespace observer.rootMargin");
+
   var rootDiv = document.getElementById("root");
   observer = new IntersectionObserver(function(e) {}, {
     root: rootDiv,