Prevent crash when page_style_sheet is missing an owner document

According to the crash report, the owner document could be a nullptr.
This CL adds a check if an owner document is available and sets the
execution_context to nullptr if it's not available.
CSSStyleDeclaration::setCSSText that receives the execution context
can handle nullptr values.

Fixed: 1090899
Change-Id: I2e0b1a56e6b25e98c043f576dbd230f50c062516
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2300382Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Commit-Queue: Alex Rudenko <alexrudenko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789404}

third_party/blink/renderer/core/inspector/inspector_style_sheet.cc

@@ -1112,8 +1112,10 @@ CSSRule* InspectorStyleSheet::SetStyleText(const SourceRange& range,
   else
     style = To<CSSKeyframeRule>(rule)->style();
 
+  Document* owner_document = page_style_sheet_->OwnerDocument();
   ExecutionContext* execution_context =
-      page_style_sheet_->OwnerDocument()->GetExecutionContext();
+      owner_document ? owner_document->GetExecutionContext() : nullptr;
+
   style->setCSSText(execution_context, text, exception_state);
 
   ReplaceText(source_data->rule_body_range, text, new_range, old_text);

third_party/blink/web_tests/inspector-protocol/css/css-set-inline-style-tag-text-expected.txt

+The test verifies functionality of protocol method CSS.setStyleTexts and DOM.undo.
+==== Initial style sheet text ====
+
+#test { color: green; }
+
+==== Style sheet text ====
+
+#test {color: blue;}
+
+Dumping matched rules: 
+*#test* {    regular
+    color: blue;
+}
+Dumping inherited rules: 
+==== Style sheet text after clearing the stylesheet and DOM.Undo ====
+<empty>
+

third_party/blink/web_tests/inspector-protocol/css/css-set-inline-style-tag-text.js

+(async function(testRunner) {
+  var {page, session, dp} = await testRunner.startHTML(`
+<style>
+#test { color: green; }
+</style>
+<article id='test'></article>
+`, 'The test verifies functionality of protocol method CSS.setStyleTexts and DOM.undo.');
+
+  const CSSHelper = await testRunner.loadScript('../resources/css-helper.js');
+  const cssHelper = new CSSHelper(testRunner, dp);
+
+  let eventPromise = dp.CSS.onceStyleSheetAdded();
+
+  await dp.DOM.enable();
+  await dp.CSS.enable();
+
+  let event = await eventPromise;
+  let styleSheetId = event.params.header.styleSheetId;
+
+  const setStyleTexts = cssHelper.setStyleTexts.bind(cssHelper, styleSheetId, false);
+  const documentNodeId = await cssHelper.requestDocumentNodeId();
+
+  let response = await dp.CSS.getStyleSheetText({styleSheetId});
+  testRunner.log('==== Initial style sheet text ====');
+  testRunner.log(response.result.text);
+
+  await setStyleTexts([{
+    styleSheetId: styleSheetId,
+    range: { startLine: 1, startColumn: 7, endLine: 1, endColumn: 22 },
+    text: "color: blue;",
+  }]);
+  await cssHelper.loadAndDumpMatchingRules(documentNodeId, '#test');
+
+  eventPromise = dp.CSS.onceStyleSheetAdded();
+  // Clear the style content.
+  await session.evaluate(fontURL => {
+    const style = document.querySelector('style');
+    style.textContent = '';
+  });
+  await dp.DOM.undo();
+
+  event = await eventPromise;
+  styleSheetId = event.params.header.styleSheetId;
+  response = await dp.CSS.getStyleSheetText({styleSheetId});
+  testRunner.log('==== Style sheet text after clearing the stylesheet and DOM.Undo ====');
+  testRunner.log(response.result.text || '<empty>');
+  testRunner.completeTest();
+})