Add bool InspectorAgentState::*Map::IsEmpty().

Will use this in the InspectorDOMDebuggerAgent and elsewhere,
since Keys() returns vector<WTF::String> and it's a bit
inefficient / ugly to call .empty() on that.

Change-Id: I99f9cb6024077bbc60ada38e46240ac602bf8669
Reviewed-on: https://chromium-review.googlesource.com/1154038Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Johannes Henkel <johannes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578885}

third_party/blink/renderer/core/inspector/inspector_session_state.h

@@ -174,6 +174,9 @@ class CORE_EXPORT InspectorAgentState {
       return keys;
     }
 
+    // O(1) shortcut for Keys().empty().
+    bool IsEmpty() const { return map_.IsEmpty(); }
+
     // Returns the value for a given |key|, or the default value if
     // the key wasn't set.
     ConstRefType Get(const WTF::String& key) const {

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

@@ -145,9 +145,13 @@ TEST(InspectorSessionStateTest, MapFields) {
     AgentWithMapFields maps_agent;
     maps_agent.agent_state_.InitFrom(&session_state);
 
+    EXPECT_TRUE(maps_agent.strings_.IsEmpty());
+
     maps_agent.strings_.Set("key1", "Hello, world.");
     maps_agent.strings_.Set("key2", WTF::String::FromUTF8("I ❤ Unicode."));
 
+    EXPECT_FALSE(maps_agent.strings_.IsEmpty());
+
     EXPECT_THAT(maps_agent.strings_.Keys(),
                 UnorderedElementsAre("key1", "key2"));
     EXPECT_EQ("Hello, world.", maps_agent.strings_.Get("key1"));